Problems creating a Popup Window in Android Activity

Question

I'm trying to create a popup window that only appears the first time the application starts. I want it to display some text and have a button to close the popup. However, I'm having troubles getting the PopupWindow to even work. I've tried two different ways of doing it:

First I have an XML file which declares the layout of the popup called popup.xml (a textview inside a linearlayout) and I've added this in the OnCreate() of my main Activity:

PopupWindow pw = new PopupWindow(findViewById(R.id.popup), 100, 100, true);
    pw.showAtLocation(findViewById(R.id.main), Gravity.CENTER, 0, 0);

Second I did the exact same with this code:

final LayoutInflater inflater = (LayoutInflater)this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    PopupWindow pw = new PopupWindow(inflater.inflate(R.layout.popup, (ViewGroup) findViewById(R.layout.main) ), 100, 100, true);
    pw.showAtLocation(findViewById(R.id.main_page_layout), Gravity.CENTER, 0, 0);

The first throws a NullPointerException and the second throws a BadTokenException and says "Unable to add window -- token null is not valid"

What in the world am I doing wrong? I'm extremely novice so please bear with me.

Answer 1

To avoid BadTokenException, you need to defer showing the popup until after all the lifecycle methods are called (-> activity window is displayed):

 findViewById(R.id.main_page_layout).post(new Runnable() {
   public void run() {
     pw.showAtLocation(findViewById(R.id.main_page_layout), Gravity.CENTER, 0, 0);
   }
});