Fix NPE in setTransparentRegion.

Check for null Surface before using it.

Fixes bug 6312835.

Change-Id: Iaaac2a5d88e81b88e369815e09818c268085e4b7
diff --git a/services/java/com/android/server/wm/WindowStateAnimator.java b/services/java/com/android/server/wm/WindowStateAnimator.java
index 220f5e0..941a5e1 100644
--- a/services/java/com/android/server/wm/WindowStateAnimator.java
+++ b/services/java/com/android/server/wm/WindowStateAnimator.java
@@ -943,6 +943,10 @@
     }
 
     void setTransparentRegionHint(final Region region) {
+        if (mSurface == null) {
+            Slog.w(TAG, "setTransparentRegionHint: null mSurface after mHasSurface true");
+            return;
+        }
         if (SHOW_LIGHT_TRANSACTIONS) Slog.i(TAG,
             ">>> OPEN TRANSACTION setTransparentRegion");
         Surface.openTransaction();