Android AsyncTask onPostExecute won't display AlertDialog

Question

I'm having an issue with my onPostExecute from an AsyncTask class. Here is the code.

--EDIT: added in the rest of the code by request--

public class MainActivity extends Activity {

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

    preliminaryTasks();
}

private void preliminaryTasks() {

    //removed more code

    new discoverAddresses().execute();

}

public class discoverAddresses extends AsyncTask<String, Void, List<String>> {

    @Override
    protected List<String> doInBackground(String... params){

        /* Lots of code here taken out */

        List<String> addresses = new ArrayList<String>();
        //Added for debugging purposes
        addresses.add("http://192.168.1.120:80");
        addresses.add("http://192.168.1.110:80");
        return addresses;

    }

    @Override
    protected void onPostExecute(List<String> addresses){

        MainActivity.this.runOnUiThread(new Runnable(){
            public void run(){
                AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
                builder.setMessage("Are you sure you want to exit?").setCancelable(false)
                        .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog, int id) {
                                MainActivity.this.finish();
                            }
                        }).setNegativeButton("No", new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog, int id) {
                                dialog.cancel();
                            }
                        });
                AlertDialog alert = builder.create();
                alert.show();
            }
        });

        AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
        builder.setMessage("Are you sure you want to exit?").setCancelable(false)
                .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int id) {
                        MainActivity.this.finish();
                    }
                }).setNegativeButton("No", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int id) {
                        dialog.cancel();
                    }
                });
        AlertDialog alert = builder.create();
        alert.show();

        return; 

    }
}
}

But the alert dialog never shows up; when I watch in the debugger it will execute the alert.show() line but it never displays on the UI. Does anyone have any ideas on why this is? Thanks!

EDIT: both AlertDialogs get executed in the debugger but never display on the UI

EDIT2: I copied just the code I posted here (to see if it was the removed code I took out for simplification that was causing the issue) into a new project and the alertdialog actually appears, meaning the code I took out is an issue. This is coming up in LogCat in my original project upon Force Close termination of the app.

02-02 11:13:28.746: D/AndroidRuntime(11658): Shutting down VM
02-02 11:13:28.746: W/dalvikvm(11658): threadid=1: thread exiting with uncaught exception (group=0x40fce300)
02-02 11:13:28.762: E/AndroidRuntime(11658): FATAL EXCEPTION: main
02-02 11:13:28.762: E/AndroidRuntime(11658): java.lang.NullPointerException
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:394)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.ArrayAdapter.getView(ArrayAdapter.java:362)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.AbsListView.obtainView(AbsListView.java:2257)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.ListView.measureHeightOfChildren(ListView.java:1244)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.ListView.onMeasure(ListView.java:1156)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.View.measure(View.java:15172)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4816)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1390)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.LinearLayout.measureVertical(LinearLayout.java:681)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.LinearLayout.onMeasure(LinearLayout.java:574)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.View.measure(View.java:15172)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4816)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1390)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.LinearLayout.measureVertical(LinearLayout.java:681)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.LinearLayout.onMeasure(LinearLayout.java:574)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.View.measure(View.java:15172)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4816)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.View.measure(View.java:15172)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4816)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.View.measure(View.java:15172)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4816)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2148)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.View.measure(View.java:15172)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1850)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1077)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1275)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4214)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:725)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.Choreographer.doCallbacks(Choreographer.java:555)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.Choreographer.doFrame(Choreographer.java:525)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:711)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.os.Handler.handleCallback(Handler.java:615)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.os.Handler.dispatchMessage(Handler.java:92)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.os.Looper.loop(Looper.java:137)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at android.app.ActivityThread.main(ActivityThread.java:4899)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at java.lang.reflect.Method.invokeNative(Native Method)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at java.lang.reflect.Method.invoke(Method.java:511)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:558)
02-02 11:13:28.762: E/AndroidRuntime(11658):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

this works for me, in OnPostExecute Method.

I add Activity object as propretie in my AsyncTask Class which I instantiate in the constructor of that AsyncTack so that I can call that activity and perform runOnUiThread in onpostexe...

AlertDialog.Builder builder = new AlertDialog.Builder(Activity);
            builder.setTitle("Connection ?");
            builder.setIcon(android.R.drawable.ic_dialog_alert);
            builder.setMessage("Test \n ");
            builder.setPositiveButton("Ok",null);
            final AlertDialog alert = builder.create();
            Activity.runOnUiThread(new java.lang.Runnable(){
                public void run(){
                    //show AlertDialog
                    alert.show();
                }
            });

Answer 2

I found the problem with my code. At some point this method, promptAddresses was being called. It has its own separate AlertDialog. Because the addressArray had an index of 5 it was screwing with the AlertDialog.builder because three of the indexes were null.

TLDR: Don't have any null values in a string array when dealing with AlertDialog.Builders

SOLUTION: final String[] addressArray = new String[addresses.size()];

Thank you very much to everyone who helped. I apologize for not giving the full code and confusing the whole problem. At any rate, your answers still helped very much.

public String promptAddresses(List<String> addresses){
    final String[] addressArray = new String[5];
    int i = 0;

    Iterator it = addresses.iterator();
    while(it.hasNext()){
        String next = (String) it.next();
        addressArray[i] = next;
        Log.i("ADDRESSES", next);
        i++;
    }

    AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
    builder.setTitle("Pick your address");
    builder.setItems(addressArray, new DialogInterface.OnClickListener() {
               public void onClick(DialogInterface dialog, int which) {
               // The 'which' argument contains the index position
               // of the selected item
           }
    });
    AlertDialog alert =  builder.create();
    alert.show();

    return hubAddress;
}