App stürzt nach zusammenführung zweier Projekte ab

package com.example.hartl.loginapp;

import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import xxxxx.loginapp.R.id.sign_up;

public class LoginActivity extends AppCompatActivity {
    TextView signup_text;
    EditText email, password;
    Button btnLogin;
    AlertDialog.Builder builder;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        signup_text = (TextView)findViewById(sign_up);
        signup_text.setOnClickListener(new View.OnClickListener() {


            @Override
            public void onClick(View v) {
                startActivity(new Intent(LoginActivity.this,RegisterActivity.class));
            }
        });
        email = (EditText) findViewById(R.id.email);
        password = (EditText) findViewById(R.id.password);

        btnLogin = (Button) findViewById(R.id.login_button);
        btnLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(email.getText().toString().equals("")||password.getText().toString().equals("")){
                    builder = new AlertDialog.Builder(LoginActivity.this);
                    builder.setTitle("Fix this Error");
                    builder.setMessage("Enter E-mail and Password");
                    builder.setPositiveButton("OK",new DialogInterface.OnClickListener(){
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            dialog.dismiss();
                        }
                   });
                    AlertDialog alertDialog = builder.create();
                    alertDialog.show();
                }else {
                    BackgroundTask backgroundTask = new BackgroundTask(LoginActivity.this);
                    backgroundTask.execute("login",email.getText().toString(),password.getText().toString());
                }
            }
        });




    }
}