How to destroy activity without crashing the app - android-studio

Question

I`v been trying for a while to destroy activity when the home button is pressed or when the app is in background and I manage to do something with this code
#Override
protected void onPause() {
this.finish();
super.onPause();
}
and when the app runs it does not crash but after a while playing in the activity the app just goes background and crashes.
This is the activity that I`m talking about:
public class Playing extends AppCompatActivity implements View.OnClickListener {
final static long INTERVAL=1154;//1 second
final static long TIMEOUT=7000;
int progressValue = 0;
CountDownTimer mcountDown;
List<Question> questionPlay = new ArrayList<>();
DbHelper db;
int index=0 , score =0,thisQuestion=0,totalQuestion,CorrectAnswer;
String mode = "";
ProgressBar progressBar;
ImageView imageView;
Button btnA, btnB, btnC,btnD;
TextView txtScore,txtQuestion;
#Override
protected void onPause() {
this.finish();
super.onPause();
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_playing);
Bundle extra = getIntent().getExtras();
if(extra!=null)
mode =extra.getString("MODE");
db = new DbHelper(this);
txtScore = (TextView)findViewById(R.id.txtScore);
txtQuestion =(TextView)findViewById(R.id.txtQuestion);
progressBar=(ProgressBar) findViewById(R.id.progreessBar);
btnA = (Button) findViewById(R.id.btnAnswerA);
btnB =(Button)findViewById(R.id.btnAnswerB);
btnC = (Button)findViewById(R.id.btnAnswerC);
btnD = (Button)findViewById(R.id.btnAnswerD);
imageView = (ImageView)findViewById(R.id.question_bike);
btnA.setOnClickListener(this);
btnB.setOnClickListener(this);
btnC.setOnClickListener(this);
btnD.setOnClickListener(this);
}
#Override
protected void onResume() {
super.onResume();
questionPlay = db.getQuestionMode(mode);
totalQuestion = questionPlay.size();
mcountDown = new CountDownTimer(TIMEOUT, INTERVAL) {
#Override
public void onTick(long millisUntilFinished) {
progressBar.setProgress(progressValue);
progressValue++;
}
#Override
public void onFinish() {
mcountDown.cancel();
showQuestion(++index);
}
};
showQuestion(index);
}
private void showQuestion(int index) {
if (index < totalQuestion){
thisQuestion++;
txtQuestion.setText(String.format("%d %d",thisQuestion,totalQuestion));
progressBar.setProgress(0);
progressValue = 0;
int ImageID = this.getResources().getIdentifier(questionPlay.get(index).getImage().toLowerCase(),"drawable",getPackageName());
imageView.setBackgroundResource(ImageID);
btnA.setText(questionPlay.get(index).getAnswerA());
btnB.setText(questionPlay.get(index).getAnswerB());
btnC.setText(questionPlay.get(index).getAnswerC());
btnD.setText(questionPlay.get(index).getAnswerD());
mcountDown.start();
}
else {
Intent intent = new Intent(this,Done.class);
Bundle dataSend = new Bundle();
dataSend.putInt("SCORE", score);
dataSend.putInt("TOTAL",totalQuestion);
dataSend.putInt("CORRECT",CorrectAnswer);
intent.putExtras(dataSend);
startActivity(intent);
finish();
}
}
#Override
public void onClick(View v) {
mcountDown.cancel();
if (index < totalQuestion){
Button clickedButton = (Button)v;
if(clickedButton.getText().equals(questionPlay.get(index).getCorrectAnswer())){
score+=1;
CorrectAnswer++;
showQuestion(++index);
}
else showQuestion(++index);
txtScore.setText(String.format("S:%d",score));
}
}
#Override
public void onBackPressed() {
Intent intent = new Intent(Playing.this ,MainActivity.class) ;
startActivity(intent) ;
finish() ;
}
}
I`v tried and with this code
#Override
protected void onDestroy(){
super.onDestroy;
finish();
}
but then the second activity does not start and the app crashes again.
So what to do?