Could someone explain to me why this null Pointer Exception?

Question

Logcat

java.lang.Nullpointerexception: Attempt to invoke virtual method 'android.text.Editable android.widget.Edittext.gettext()' on a null Object Reference at com.paivadeveloper.megamessenger.Registeractivity$1.onClick(Registeractivity.java:49) at android.view.View.performClick(View.java:6261) at android.widget.Textview.performClick(Textview.java:11159) at android.view.View$Performclick.run(View.java:23748) 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:6776) at java.lang.reflect.Method.invoke(Native Method) at com.android.Internal.os.Zygoteinit$Methodandargscaller.run(Zygoteinit.java:1496) at com.android.Internal.os.Zygoteinit.main(Zygoteinit.java:1386)

Java

public class RegisterActivity extends AppCompatActivity {
private FirebaseAuth mAuth;


private TextInputLayout mDisplayName;
private TextInputLayout mEmail;
private TextInputLayout mPassword;
private Button mCreateBtn;

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

    mAuth = FirebaseAuth.getInstance();

    mDisplayName = (TextInputLayout) findViewById(R.id.reg_display_name);
    mEmail = (TextInputLayout) findViewById(R.id.reg_email);
    mPassword = (TextInputLayout) findViewById(R.id.reg_password);
    mCreateBtn = (Button) findViewById(R.id.reg_create_btn);


    mCreateBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            String display_name = mDisplayName.getEditText().getText().toString();
            String email = mEmail.getEditText().getText().toString();
            String password = mPassword.getEditText().getText().toString();

            register_user(display_name, email, password);
        }
    });

}

private void register_user(String display_name, String email, String password) {

    mAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
        @Override
        public void onComplete(@NonNull Task<AuthResult> task) {
            if (task.isSuccessful()) {
                Intent mainIntent = new Intent(RegisterActivity.this, MainActivity.class);
                startActivity(mainIntent);
                finish();
            } else {
                Toast.makeText(RegisterActivity.this, "Erro , contate-nos", Toast.LENGTH_LONG).show();
            }
        }
    });
    }

    }

Thank you