尝试在空对象引用上调用虚拟方法'android.text.Editable android.widget.EditText.getText（）'时出错

Question

我正在开发一个应用程序，并且在构建登录活动时，每当我按下LOGIN按钮并显示此错误时，该应用程序便崩溃了。这只是使用firebase的简单登录活动。

这是登录活动的XML代码，

    <?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity"
android:id="@+id/relativeLayout"
android:background="#000000">

<TextView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="124dp"
    android:text="EMAIL"
    android:textSize="15dp"
    android:layout_marginLeft="15dp"
    android:layout_marginRight="15dp"
    android:textColor="#FFFFFF"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    />

<EditText
    android:id="@+id/name_textview"
    android:layout_width="match_parent"
    android:layout_height="40dp"
    android:layout_marginTop="160dp"
    android:layout_marginLeft="15dp"
    android:layout_marginRight="15dp"
    android:textColor="#FFFFFF"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="16dp"
    android:layout_marginStart="16dp"
    android:layout_marginTop="224dp"
    android:text="PASSWORD"
    android:textColor="#FFFFFF"
    android:textSize="15dp"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<EditText
    android:id="@+id/password_textview"
    android:layout_width="match_parent"
    android:layout_height="40dp"
    android:layout_marginTop="260dp"
    android:layout_marginLeft="15dp"
    android:layout_marginRight="15dp"
    android:textColor="#FFFFFF"
    android:inputType="textPassword"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<Button
    android:id="@+id/login_button"
    android:layout_width="352dp"
    android:layout_height="40dp"
    android:layout_marginLeft="16dp"
    android:layout_marginStart="16dp"
    android:layout_marginTop="332dp"
    android:text="L O G I N"
    android:textColor="#FFFFFF"
    android:background="#404040"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<TextView
    android:id="@+id/registration_text"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="112dp"
    android:layout_marginStart="112dp"
    android:layout_marginTop="448dp"
    android:text="New User? Register Here"
    android:textColor="#FFFFFF"
    android:textSize="15dp"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

＆这是登录活动的JAVA代码，

package com.example.theroos.loginapp;

import android.app.ProgressDialog;
import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
public class MainActivity extends AppCompatActivity {

EditText enter_email, enter_password;
Button login_button;
TextView gotoreg_text;
FirebaseAuth firebaseAuth;
ProgressDialog progressDialog;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    enter_email=(EditText)findViewById(R.id.email_edittext);
    enter_password=(EditText)findViewById(R.id.Password_editText);
    login_button=(Button)findViewById(R.id.login_button);
    gotoreg_text=(TextView)findViewById(R.id.registration_text);
    firebaseAuth=FirebaseAuth.getInstance();
    FirebaseUser user = firebaseAuth.getCurrentUser();
    progressDialog = new ProgressDialog(this);

    if (user != null){
        finish();
        startActivity(new Intent(MainActivity.this,DestinationActivity.class));
    }

    login_button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            validate(enter_email.getText().toString(),enter_password.getText().toString());
        }
    });

    gotoreg_text.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivity(new Intent(MainActivity.this,RegistrationActivity.class));
        }
    });
}

private void validate (String username, String userpassword){

    progressDialog.setMessage("wait for a while");
    progressDialog.show();;

    firebaseAuth.signInWithEmailAndPassword(username,userpassword).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
        @Override
        public void onComplete(@NonNull Task<AuthResult> task) {
            if (task.isSuccessful()){
                progressDialog.dismiss();
                Toast.makeText(MainActivity.this, "Successfully logged in", Toast.LENGTH_SHORT).show();
                startActivity(new Intent(MainActivity.this,DestinationActivity.class));
            }
            else {
                progressDialog.dismiss();
                Toast.makeText(MainActivity.this, "You have entered wrong email or password", Toast.LENGTH_SHORT).show();
            }
        }
    });


}
}

＆这是我在logcat中遇到的错误，

FATAL EXCEPTION: main
Process: com.example.theroos.loginapp, PID: 31527
java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference
    at com.example.theroos.loginapp.MainActivity$1.onClick(MainActivity.java:51)
    at android.view.View.performClick(View.java:5609)
    at android.view.View$PerformClick.run(View.java:22263)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6077)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

如果您需要帮助我找出该问题，我将不胜感激，我无法弄清login_button.setOnClickListener()的问题。提前致谢。