Proses Pemanggilan Proses Login Pada MainActivity

package com.example.lenovo.myapplication;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
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.TextView;

import com.example.lenovo.myapplication.proses.proses_login;

public class WelcomeActivity extends AppCompatActivity {

    EditText username, password;
    Button btn_login;
    TextView yuk;
    AlertDialog.Builder builder;

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

        username = (EditText) findViewById(R.id.editText);
        password = (EditText) findViewById(R.id.editText2);
        btn_login = (Button) findViewById(R.id.button);
        yuk = (TextView) findViewById(R.id.yuk_daftar);
        yuk.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(WelcomeActivity.this, FormRegister.class);
                startActivity(intent);
            }
        });
        btn_login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (username.getText().toString().equals("")||password.getText().toString().equals("")){
                    builder = new AlertDialog.Builder(WelcomeActivity.this);
                    builder.setTitle("Warning..!");
                    builder.setMessage("Your fill not input..!!");
                    builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialogInterface, int i) {
                            dialogInterface.dismiss();
                        }
                    });
                    AlertDialog alertDialog = builder.create();
                    alertDialog.show();
                }
                else {
                    proses_login backgroundTask = new proses_login(WelcomeActivity.this);
                    backgroundTask.execute("Login", username.getText().toString(),password.getText().toString());
                }
            }
        });

    }
}