Fix NPE in PhotoPage

Bug: 7673141
Change-Id: I264f28d127f47e5f4f8c6a7c2df1c021401d03f2
diff --git a/src/com/android/gallery3d/app/PhotoPage.java b/src/com/android/gallery3d/app/PhotoPage.java
index b43cf2a..506d1ca 100644
--- a/src/com/android/gallery3d/app/PhotoPage.java
+++ b/src/com/android/gallery3d/app/PhotoPage.java
@@ -304,7 +304,9 @@
                         break;
                     }
                     case MSG_ON_FULL_SCREEN_CHANGED: {
-                        mAppBridge.onFullScreenChanged(message.arg1 == 1);
+                        if (mAppBridge != null) {
+                            mAppBridge.onFullScreenChanged(message.arg1 == 1);
+                        }
                         break;
                     }
                     case MSG_UPDATE_ACTION_BAR: {