My Android app is crashing after loggin screen

Crash:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.online_exam.quizapp, PID: 15277
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.online_exam.quizapp/com.example.online_exam.quizapp.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2740)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2801)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1540)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:163)
        at android.app.ActivityThread.main(ActivityThread.java:6358)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:909)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:799)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.online_exam.quizapp.LoginActivity.onCreate(LoginActivity.java:53)
        at android.app.Activity.performCreate(Activity.java:6840)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2693)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2801) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1540) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:163) 
        at android.app.ActivityThread.main(ActivityThread.java:6358) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:909) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:799) 
I/Process: Sending signal. PID: 15277 SIG: 9 
Process 15277 terminated.

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This is my Login Activity. In my app it starts up to login activity and after I login the app crashes suddenly.

EditText emailId, password;
Button btnSignIn;
TextView tvSignUp;
FirebaseAuth mFirebaseAuth;
private FirebaseAuth.AuthStateListener mAuthStateListener;

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

    mFirebaseAuth = FirebaseAuth.getInstance();
    emailId = (EditText) findViewById(R.id.editText);
    password = (EditText) findViewById(R.id.editText2);
    btnSignIn = (Button) findViewById(R.id.button2);
    tvSignUp = (TextView) findViewById(R.id.textView);

    mAuthStateListener = new FirebaseAuth.AuthStateListener() {
        @Override
        public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
            FirebaseUser mFirebaseUser = mFirebaseAuth.getCurrentUser();
            if (mFirebaseUser != null) {
                Toast.makeText(LoginActivity.this, "You Are Logged In", Toast.LENGTH_SHORT).show();
                Intent i = new Intent(LoginActivity.this, MainActivity.class);
                startActivity(i);
            } else {
                Toast.makeText(LoginActivity.this, "Please Login", Toast.LENGTH_SHORT).show();
            }

        }
    };

    btnSignIn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String email = emailId.getText().toString();
            String pwd = password.getText().toString();
            if (email.isEmpty()) {
                emailId.setError("Please enter email");
                emailId.requestFocus();
            } else if (pwd.isEmpty()) {
                password.setError("please enter your password");
                password.requestFocus();
            } else if (email.isEmpty() && pwd.isEmpty()) {
                Toast.makeText(LoginActivity.this, "Fields are Empty", Toast.LENGTH_SHORT).show();
            } else if (!(email.isEmpty() && pwd.isEmpty())) {
                mFirebaseAuth.signInWithEmailAndPassword(email, pwd).addOnCompleteListener(LoginActivity.this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (!task.isSuccessful()) {
                            Toast.makeText(LoginActivity.this, "Login error", Toast.LENGTH_SHORT).show();
                        } else {
                            Intent intToMain = new Intent(LoginActivity.this, MainActivity.class);
                            startActivity(intToMain);
                        }
                    }
                });
            } else {
                Toast.makeText(LoginActivity.this, "Error Occured!", Toast.LENGTH_SHORT).show();
            }
        }
    });

    tvSignUp.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intSignup = new Intent(LoginActivity.this, SignupActivity.class);
            startActivity(intSignup);
        }
    });
}

@Override
protected void onStart() {
    super.onStart();
    mFirebaseAuth.addAuthStateListener(mAuthStateListener);
}

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This is my activity of login. I don't know what the error is coming from.