Android Studio Registrierungs-Activity mit FireBase - App startet nicht

package net.stelan.registrierung;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import net.stelan.registrierung.Model.User;
public class MainActivity extends AppCompatActivity {
    //String Variable
    String UsernameExist;
    String Success;
    //FireBase
    FirebaseDatabase database;
    DatabaseReference users;
    EditText edtUsername, edtPassword, edtMail;
    Button btnSingUp;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        UsernameExist = getString(R.string.UsernameExist);
        Success = getString(R.string.Success);
        //FireBase
        database = FirebaseDatabase.getInstance();
        users = database.getReference("Users");
        edtUsername = (EditText) findViewById(R.id.edtUsername);
        edtPassword= (EditText) findViewById(R.id.edtPassword);
        edtMail = (EditText) findViewById(R.id.edtMail);
        btnSingUp = (Button) findViewById(R.id.btnSingUp);
        btnSingUp.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final User user = new User(edtUsername.getText().toString(),
                        edtPassword.getText().toString(),
                        edtMail.getText().toString());
                users.addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        if (dataSnapshot.child(user.getUsername()).exists())
                            Toast.makeText(MainActivity.this, UsernameExist, Toast.LENGTH_LONG).show();
                        else {
                            users.child(user.getUsername()).setValue(user);
                            Toast.makeText(MainActivity.this, Success, Toast.LENGTH_SHORT).show();
                        }
                    }
                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {
                        //add
                    }
                });
            }
        });
    }
}