收到类似'console.error：“ Error”：，{}的错误

Question

在React本机中创建一个项目，对于登录页面，我从url中获取json数据。为此，我编写了这样的代码，

import React, {Component} from 'react';
import { StyleSheet,
    Text,
    View,
    TextInput,
    TouchableHighlight,
    Alert,
    TouchableOpacity,
} from 'react-native';

import Icon from 'react-native-vector-icons/FontAwesome';

export default class Loginscreen extends Component{
    constructor() {
        super();
        this.state = {
          username   : '',
          password : '',
        }
      }

    navigateToHomescreen = () => {
        this.props.navigation.navigate('Homescreen');
      }
    submit()
    {
      let collection={ }
      collection.email=this.state.email,
      collection.password=this.state.password
    //   console.warn(collection);
      var url = 'my url';

      fetch(url, {
      method: 'POST', // or 'PUT'
      body: JSON.stringify(collection), // data can be `string` or {object}!
      headers: new Headers({
     'Content-Type': 'application/json'
      })
      }).then(res => res.json())
      .catch(error => console.error('Error:', error))
      .then(response => console.log('Success:',response));// this._navigateToHomescreen 


    }


    onClickListener = (viewId) => {
        Alert.alert("Alert", "Button pressed "+viewId);

    }
    render() {
        return (
          <View style={styles.container}>
            <View>
              {/* <FontAwesome>{Icons.chevronLeft}</FontAwesome> */}

              <Text style={styles.inputHeading}>Staff Login</Text>
            </View>
            <View style={styles.inputContainer}>
              <Icon style={styles.inputIcon} name='user' size={30} ></Icon>
              <TextInput style={styles.inputs}
                  placeholder="Username"
                  keyboardType="email-address"
                  underlineColorAndroid='transparent'
                  onChangeText={(email) => this.setState({email})}
                  />
            </View>


            <View style={styles.inputContainer}>
            <Icon style={styles.inputIcon} name='key' size={30} ></Icon>
              <TextInput style={styles.inputs}
                  placeholder="Password"
                  secureTextEntry={true}
                  underlineColorAndroid='transparent'
                  onChangeText={(password) => this.setState({password})}
                  />
            </View>

            <TouchableOpacity style={[styles.buttonContainer, styles.loginButton]} onPress={() => this.submit()}>
              <Text style={styles.loginText}>Login</Text>
            </TouchableOpacity>

            <TouchableHighlight style={styles.buttonContainer} onPress={() => this.onClickListener('restore_password')}>
                <Text>Forgot your password?</Text>
            </TouchableHighlight>
          </View>
        );
      }
    }

在执行此操作时会收到类似'console.error：“ Error”：，{}的错误 请任何人帮助我，我陷入了这个错误。还告诉我如何从登录页面导航到主页。