Firebase Auth - LogIn Button unsichtbar

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
    android:background="@drawable/tafel2"
    tools:context=".MainActivity">


    <Button
        android:id="@+id/startbutton"
        android:layout_width="200dp"
        android:layout_height="70dp"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginTop="150dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        android:background="@drawable/startbutton1"
        android:textSize="24sp"
        android:textStyle="bold"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />    


    <Button
        android:id="@+id/btn_sign_out"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="70dp"
        android:enabled="false"
        android:text="@string/signout"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/rubrikbutton" />


</androidx.constraintlayout.widget.ConstraintLayout>

package ...;


import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;


import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;


import com.firebase.ui.auth.AuthUI;
import com.firebase.ui.auth.IdpResponse;
import com.google.android.gms.auth.api.Auth;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;


import java.util.Arrays;
import java.util.List;


public class MainActivity extends AppCompatActivity implements View.OnClickListener {


    private static final int MY_REQUEST_CODE = ...;
    List<AuthUI.IdpConfig> providers;
    Button btn_sign_out;


    private FirebaseDatabase database = FirebaseDatabase.getInstance();
    private DatabaseReference myRef = database.getReference();


    Button startbutton;


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


        btn_sign_out = (Button) findViewById(R.id.btn_sign_out);
        btn_sign_out.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //Logout
                AuthUI.getInstance()
                        .signOut(MainActivity.this)
                        .addOnCompleteListener(new OnCompleteListener<Void>() {
                            @Override
                            public void onComplete(@NonNull Task<Void> task) {
                                btn_sign_out.setEnabled(false);
                                showSignInOptions();
                            }
                        }).addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        Toast.makeText(MainActivity.this, ""+e.getMessage(), Toast.LENGTH_SHORT).show();


                    }
                });
            }
        });


        providers = Arrays.asList(
                new AuthUI.IdpConfig.EmailBuilder().build(),
                new AuthUI.IdpConfig.PhoneBuilder().build(),
                new AuthUI.IdpConfig.FacebookBuilder().build(),
                new AuthUI.IdpConfig.GoogleBuilder().build()
        );


        showSignInOptions();
    }


    private void showSignInOptions() {
        startActivityForResult(
                AuthUI.getInstance().createSignInIntentBuilder()
                .setAvailableProviders(providers)
                .setTheme(R.style.MyTheme)
                .build(), MY_REQUEST_CODE
        );
    }


    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == MY_REQUEST_CODE)
        {
            IdpResponse response = IdpResponse.fromResultIntent(data);
            if (resultCode == RESULT_OK){
                //Get User
                FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
                //Show E-Mail on toast
                Toast.makeText(this, ""+user.getEmail(), Toast.LENGTH_SHORT).show();
                //Set button Sign-Out
                btn_sign_out.setEnabled(true);
            }
            else
            {
                Toast.makeText(this, ""+response.getError().getMessage(), Toast.LENGTH_SHORT).show();
            }
        }
    }
}