AlertDialog 대화상자 도와주세요

Question

package com.example.a.alertdialog

import android.app.Activity;
import android.content.DialogInterface;
import android.os.Bundle;
import android.preference.DialogPreference;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.Toast;
import android.app.AlertDialog;
import android.view.View;

public class MainActivity extends Activity {

    RadioGroup rGroup1;
    RadioButton rdoDog, rdoCat, rdoRabbit,rdoHorse;
    Button btnView;
    ImageView imgPet;
    View dialogView;

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

        dialogView = (View)View.inflate(MainActivity.this,R.layout.image,null);

        rGroup1 = (RadioGroup) findViewById(R.id.Rg1);
        rdoDog = (RadioButton) findViewById(R.id.RdoDog);
        rdoCat = (RadioButton) findViewById(R.id.RdoCat);
        rdoRabbit = (RadioButton) findViewById(R.id.RdoRabbit);
        rdoHorse = (RadioButton) findViewById(R.id.RdoHorse);
        btnView = (Button) findViewById(R.id.BtnView);
        imgPet = (ImageView)dialogView.findViewById(R.id.ImgPet);

 

          btnView.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View arg0) {
                AlertDialog.Builder dlg = new AlertDialog.Builder(
                        MainActivity.this);
                switch (rGroup1.getCheckedRadioButtonId()) {
                    case R.id.RdoDog:
                        dlg.setTitle("개");
                        dlg.setView(dialogView);
                        imgPet.setImageResource(R.drawable.dog);
                        dlg.setPositiveButton("닫기", null);
                        dlg.show();
                        break;
                    case R.id.RdoCat:

                        dlg.setTitle("고양이");
                        dlg.setView(dialogView);
                        imgPet.setImageResource(R.drawable.cat);
                        dlg.setPositiveButton("닫기", null);
                        dlg.show();
                        break;
                    case R.id.RdoRabbit:

                        dlg.setTitle("토끼");
                        dlg.setView(dialogView);
                        imgPet.setImageResource(R.drawable.rabbit);
                        dlg.setPositiveButton("닫기", null);
                        dlg.show();
                        break;
                    case R.id.RdoHorse:

                        dlg.setTitle("말");
                        dlg.setView(dialogView);
                        imgPet.setImageResource(R.drawable.horse);
                        dlg.setPositiveButton("닫기", null);
                        dlg.show();
                        break;
                    default:
                        Toast.makeText(getApplicationContext(), "동물 먼저 선택하세요", Toast.LENGTH_SHORT)
                                .show();
                }

            }
        });

    }
}

라디오 버튼 체크후 버튼을 누르면 이미지가 나오는 대화상자를 만들고싶은데

이렇게 하게 되면 맨 처음에 할때만 이미지가 나오고 대화상자 닫기 누르고 다시 다른걸 누르면 어플이 그냥 꺼져요....

코딩이 어디가 잘못된건지 도와주세요