bug #6176711: guard against null GalleryActionBar.

When entering Gallery app from WallPaper picker, the activity is DialogPicker, which doesn't initialize GalleryActionbar so getGalleryActionbar() returns null. Guard against using the null object.

I fixed the obvious NPE here, but Ray or Yuli should take a closer look, to see why this is broken.

Change-Id: Ic6923c388ea7b72ab402131fc3b039546509e340