ListView is not inside R.layout.activity_main
so it gives you null
You need change these line
ListView lv=(ListView)findViewById(R.id.listView1);
ArrayAdapter<String> aa=new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1, myList);
lv.setAdapter(aa);
here it gives you NullPointerException
..
so you can change it like inside your onClick()
method..
Dialog dialog=new Dialog(MainActivity.this);
dialog.setContentView(R.layout.dialog_layout);
ListView lv=(ListView)dialog.findViewById(R.id.listView1);
ArrayAdapter<String> aa=new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, myList);
lv.setAdapter(aa);
dialog.show();