TypeError：无法读取null的属性“ seq”

Question

我有连接到服务器的数据库和连接到客户端的服务器，但是尝试注册时出现错误。我的客户注册文件：

import React, {Component} from 'react';
import PropTypes from 'prop-types';
import getMuiTheme from 'material-ui/styles/getMuiTheme';
import {withRouter} from 'react-router-dom';
import {connect} from 'react-redux';
import RaisedButton from 'material-ui/RaisedButton';
import {signError, signUp} from '../actions';
import $ from 'jquery';

import path from 'path';
import TextInput from '../components/text-input';

import Header from '../components/header';
import Footer from '../components/footer';
class Signup extends Component {
    constructor(props) {
        super(props);
        this.state = {
            muiTheme: getMuiTheme(),
            dialogStyle: {display: 'none'},
            isSubmitting: false
        };
    }

    static childContextTypes = {
        muiTheme: PropTypes.object
    };

    getChildContext() {
        return {muiTheme: this.state.muiTheme};
    }

    static propTypes = {
        topOffset: PropTypes.number,
        leftOffset: PropTypes.number
    };

    componentDidMount() {
        this.setState({
            dialogStyle: {
                display: 'flex',
                justifyContent: 'center',
                marginTop: 0,
                marginBottom: 0,
                marginLeft: 0,
                marginRight: 0,
                top: this.props.topOffset,
                left: this.props.leftOffset
            }
        });

        $('#avatar_image').hide();
    }

    componentWillMount() {
        this.props.signError('');
    }

    submitForm = (e) => {
        e.preventDefault();

        const {isSubmitting} = this.state;

        if(isSubmitting) return;

        const email = $('#email').val();
        const password = $('#password').val();
        const passwordConfirm = $('#passwordConfirm').val();
        const name = $('#name').val();
        const fileToUpload = document.getElementById('fileToUpload').files;

        if (email && password && passwordConfirm && name) {
            if (email.length > 0 && password.length > 0 && passwordConfirm.length > 0 && name.length > 0) {
                if(password === passwordConfirm) {
                    if(!isSubmitting) {
                        $('#submit').html('<img src="/public/assets/images/spinner.gif"/>');
                        this.setState({isSubmitting:true});
                    }

                    const failed = () => {
                        $('#submit').html('Submit');
                        this.setState({isSubmitting:false});
                    };

                    let avatar = null;

                    if (fileToUpload) {
                        if (fileToUpload[0]) {
                            const filename = fileToUpload[0].name;

                            if (filename && filename.length > 0) {
                                const ext = path.extname(filename).toLowerCase();

                                if (ext === '.png' || ext === '.jpg' || ext === '.jpeg' || ext === '.gif' || ext === '.ico') {
                                    avatar = fileToUpload[0];
                                }
                            }
                        }
                    }

                    const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
                    return sleep(300).then(() => {
                        this.props.signUp({email, password, name, avatar, failed});
                    });
                }
            }
        }
    };

    signError = () => {
        if (this.props.error) {
            return (
                <div className="alert alert-danger">
                    <strong>{this.props.error}</strong>
                </div>
            );
        }
    };

    uploadFile(event) {
        const file = event.target.files[0];
        if(file) {
            const ext = path.extname(file.name).toLowerCase();

            if (ext === '.png' || ext === '.jpg' || ext === '.jpeg' || ext === '.gif' || ext === '.ico') {
                let reader = new FileReader();

                reader.onload = function (e) {
                    const avatar_image = $('#avatar_image');
                    avatar_image.attr('src', e.target.result);
                    const avatar_text = $('#avatar_text');
                    avatar_text.attr('placeholder', file.name);
                    avatar_image.show();
                };

                reader.readAsDataURL(file);
            }
        }
        else {
            $('#avatar_image').hide();
        }
    }

    renderButton = () => {
        return (
            <div style={{display: 'flex', justifyContent:'left', alignItems:'center', width:'100%', height:'60px'}}>
                <RaisedButton
                    label="Avatar Image"
                    labelPosition="before"
                    style={{marginLeft: 0, marginRight:20}}
                    containerElement="label"
                >
                    <input
                        type="file"
                        style={{
                            cursor: 'pointer',
                            position: 'absolute',
                            top: 0,
                            bottom: 0,
                            right: 0,
                            left: 0,
                            width: '100%',
                            opacity: 0,
                            marginRight: 20
                        }}
                        id="fileToUpload"
                        name="fileToUpload"
                        className="form-control"
                        onChange={this.uploadFile}/>
                </RaisedButton>
                <div>
                    <input id="avatar_text" type="text" readOnly="" className="form-control" placeholder=""/>
                </div>
                <div>
                    <img id="avatar_image" width="50px" height="50px" alt="" className="img-circle" style={{marginLeft:4,marginRight:4, verticalAlign:'middle'}}/>
                </div>
            </div>
        );
    };

    validateEmail = (value) => {
        let error = '';
        if (!value || value.length <= 0) {
            error = 'Required';
        } else if (!/^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i.test(value)) {
            error = 'Invalid email address';
        }

        return error;
    };

    validateName = (value) => {
        let error = '';
        if (!value || value.length <= 0) {
            error = 'Required';
        } else if (value.length < 4) {
            error = 'Must be 4 characters or more';
        } else if (value.length > 30) {
            error = 'Must be 30 characters or less';
        }

        return error;
    };

    validatePassword = (value) => {
        let error = '';
        if (!value || value.length <= 0) {
            error = 'Required';
        } else if (value.length < 4) {
            error = 'Must be 4 characters or more';
        } else if (value.length > 15) {
            error = 'Must be 15 characters or less';
        }

        return error;
    };

    validatePasswordConfirm = (value) => {
        let error = '';
        if (!value || value.length <= 0) {
            error = 'Required';
        }

        const password = $('#password').val();

        if (value !== password) {
            error = 'Passwords must match';
        }

        return error;
    };

    reset = () => {
        this._email.reset();
        this._password.reset();
        this._passwordConfirm.reset();
        this._name.reset();

        $('#fileToUpload').val('');
        $('#avatar_image').attr('src', '').hide();
        $('#avatar_text').attr('placeholder', '');
    };

    render() {
        return (
            <div>
                <Header/>
                <div style={{textAlign: 'center'}}>
                    {this.signError()}
                </div>
                <div style={this.state.dialogStyle}>
                    <form style={{width:'60%', height:'100%',
                    marginTop:20, marginBottom:100}}
                        id='form_info'
                        name='form_info'
                        onSubmit={(e) => this.submitForm(e)}>
                        <TextInput
                            ref={e=>this._email = e}
                            label="Email"
                            name="email"
                            type="text"
                            validate={this.validateEmail}
                        />
                        <TextInput
                            ref={e=>this._password = e}
                            label="Password"
                            name="password"
                            type="password"
                            validate={this.validatePassword}
                            placeholder="Please enter a password"
                        />
                        <TextInput
                            ref={e=>this._passwordConfirm = e}
                            label="Confirm Password"
                            name="passwordConfirm"
                            type="password"
                            validate={this.validatePasswordConfirm}
                            placeholder="Please re-type password"
                        />
                        <TextInput
                            ref={e=>this._name = e}
                            label="Name"
                            name="name"
                            type="text"
                            validate={this.validateName}
                            placeholder="Please enter your name"
                        />
                        <div className="form-group">
                            {this.renderButton()}
                        </div>
                        <div style={{clear:'both'}}>&nbsp;</div>
                        <div style={{display:'flex', justifyContent:'center'}}>
                            <button
                                id="submit"
                                type="submit"
                                value="Submit"
                                name="submit"
                                className="btn btn-lg btn-primary">Submit</button>
                            &nbsp;&nbsp;&nbsp;
                            <button
                                type="button"
                                value="Clear"
                                name="clear"
                                className="btn btn-lg btn-default"
                                onClick={(e) => {
                                    e.preventDefault();
                                    this.props.signError('');
                                    this.reset();
                                }}>Clear</button>
                        </div>
                    </form>
                </div>
            <Footer/>
            </div>
        );
    }
}

function mapStateToProps(state) {
    return {
        error: state.auth.error
    };
}

const mapDispatchToProps = dispatch => {
    return {
        signError: (error) => dispatch(signError(error)),
        signUp: ({email, password, name, avatar, failed}) => dispatch(signUp({email, password, name, avatar, failed}))
    }
};

export default Signup = withRouter(connect(mapStateToProps, mapDispatchToProps)(Signup));

但是提交后，我在Web控制台“ xhr.js：178 POST http://localhost:5000/signup net :: ERR_CONNECTION_RESET”中遇到错误

服务器控制台中的错误：“ TypeError：无法读取null的属性'seq'
在C：/projekti/iStudy/iStudy-Server/models/user.js：39：25
在model.Query。 （C：\ projekti \ iStudy \ iStudy-
服务器\ node_modules \猫鼬\ lib \ model.js：4093：16） 在C：\ projekti \ iStudy \ iStudy-Server \ node_modules \ kareem \ index.js：273：21
在C：\ projekti \ iStudy \ iStudy-Server \ node_modules \ kareem \ index.js：131：16
在_combinedTickCallback（内部/进程/next_tick.js:132:7）
在process._tickDomainCallback（internal / process / next_tick.js：219：9）

还有我的user.js

import mongoose from 'mongoose';
import {counter} from './sequence';
const Schema = mongoose.Schema;
const ObjectId = Schema.Types.ObjectId;
const UserSchema = new Schema({
    no: Number,
    email: {type: String, unique: true, lowercase: true},
    password: {type: String, default: ''},
    profile: {
        name: {type: String, default: ''},
        picture: {type: String, default: ''},
        instructor: {type: Boolean, default: false}
    },
    courses: [{
        ref: { type: ObjectId, ref: 'Course'},
        no: { type: Number, default: 0},
        learn: { type: Boolean, default: false}
    }],
    google: {
        type: Object
    },
    facebook: {
        type: Object
    },
    twitter: {
        type: Object
    }
});

UserSchema.pre('save', function (next) {
    const doc = this;
    const name = 'user_counter';

    counter.findByIdAndUpdate({_id: name}, {$inc: { seq: 1} }, function(err, result) {
        if(err) {

            return next(err);
        }
        doc.no = result.seq;
        next();
    });
});

export default mongoose.model('User', UserSchema, 'users');