Protect against weird null case

No time to figure out what's going on, but this null check helps

Test: Dock app, make fullscreen, hammer home button
Change-Id: I3eb94a8138bc1483b1711556696bcbbaee6084e7
Bug: 36339388

services/core/java/com/android/server/am/ActivityMetricsLogger.java

@@ -251,7 +251,8 @@ class ActivityMetricsLogger {
      *                       ActivityManagerInternal.APP_TRANSITION_* reasons.
      */
     void notifyTransitionStarting(SparseIntArray stackIdReasons) {
-        if (!isAnyTransitionActive() || mLoggedTransitionStarting) {
+        // TODO (b/36339388): Figure out why stackIdReasons can be null
+        if (stackIdReasons == null || !isAnyTransitionActive() || mLoggedTransitionStarting) {
             return;
         }
         mCurrentTransitionDelayMs = calculateCurrentDelay();