extensionPrefs: Add missing return value

When commit a7ec7583aa merged two functions into one, it left
out the expected return value from one branch.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/635

js/extensionPrefs/main.js

@@ -78,6 +78,8 @@ var Application = GObject.registerClass({
         dialog.set_default_size(600, 400);
         dialog.add(widget);
         dialog.show();
+
+        return true;
     }
 
     _buildErrorUI(row, exc) {