Android Studio: Session 'MainActivity': error, but no error

Hi I have been racking my brains for hours doing research on the internet but no one seems to get an answer. My emulator ran my code without issue, then I ran it again and I got "Session" MainActivity: error ". I looked at this main activity about 10 times but there is no error sign anywhere and it looks like it should work fine. i mean it was working before there were no problems so i'm not sure if the problem really is, i can't see if android studio is not pointing correctly or if it is a different problem altogether.

Any help would be greatly appreciated. Thank.

package tekvision.codedecrypter;
import android.content.Context;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.View;
import android.view.WindowManager;
import android.widget.Button;
import android.widget.Toast;
import gameInfo.GameDatabase;

public class MainActivity extends ActionBarActivity {

    //Runs before the application is created
    private Button mCampaignButton;
    private final Context context = this;

    //When the application is created
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

    //Instantiate a GameDatabase object (this activity)
    final GameDatabase gDB = new GameDatabase(context);

    gDB.fillGameDatabase();

    //Keeps screen on so it doesn't fall asleep
    getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);

    //Finding button by button id after application is created
    mCampaignButton = (Button)findViewById(R.id.campaignButtonID);

    //Checks if the campaign button is clicked
    mCampaignButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            String ans =  gDB.getAnswer("Ancient",1);

            //Toast pop up message
            Toast toast = Toast.makeText(getApplicationContext(),
                 ans ,
                    Toast.LENGTH_SHORT);

            toast.show();


            //Intent to go from main activity to campaign Level Select Activity

            final Intent intent = new Intent(MainActivity.this, CampaignSelectLevel.class);

            startActivity(intent);

        }
    });

}

      

        
        
        
      

    

}