show an alert dialog in broadcast receiver after a system reboot

The problem is you are trying to show an AlertDialog from a BroadcastReceiver, which isn’t allowed. You can’t show an AlertDialog from a BroadcastReceiver. Only activities can display dialogs.

You should do something else, have the BroadcastReceiver start on boot as you do and start an activity to show the dialog.

Here is a blog post more on this.

EDIT:

Here is how I would recommend doing it. From your BroadcastReceiver start an Activity with an AlertDialog as such..

public class NotifySMSReceived extends Activity 
{
    private static final String LOG_TAG = "SMSReceiver";
    public static final int NOTIFICATION_ID_RECEIVED = 0x1221;
    static final String ACTION = "android.provider.Telephony.SMS_RECEIVED";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        IntentFilter filter = new IntentFilter(ACTION);
        this.registerReceiver(mReceivedSMSReceiver, filter);
    }

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

    private final BroadcastReceiver mReceivedSMSReceiver = new BroadcastReceiver() {

        @Override
        public void onReceive(Context context, Intent intent) {
            String action = intent.getAction();

            if (ACTION.equals(action)) 
            {
                //your SMS processing code
                displayAlert();
            }
        }
    }
}

As you see here I NEVER called setContentView(). This is because the activity will have a transparent view and only the alert dialog will show.