Fix NPE in Live Wallpaper loader

Bug: 11584868
diff --git a/src/com/android/launcher3/LiveWallpaperListAdapter.java b/src/com/android/launcher3/LiveWallpaperListAdapter.java
index 54e0af7..152a75d 100644
--- a/src/com/android/launcher3/LiveWallpaperListAdapter.java
+++ b/src/com/android/launcher3/LiveWallpaperListAdapter.java
@@ -190,7 +190,9 @@
                     LiveWallpaperListAdapter.this.notifyDataSetChanged();
                     break;
                 }
-                info.mThumbnail.setDither(true);
+                if (info.mThumbnail != null) {
+                    info.mThumbnail.setDither(true);
+                }
                 if (mWallpaperPosition < mWallpapers.size()) {
                     mWallpapers.set(mWallpaperPosition, info);
                 } else {