Skip setting the controller if the animation is already finished

Bug: 154568748
Change-Id: I999c0cc3a4d9661bcf6dcd1e804746a3a0959a83
diff --git a/quickstep/src/com/android/quickstep/TaskAnimationManager.java b/quickstep/src/com/android/quickstep/TaskAnimationManager.java
index f5088e7..21e8c92 100644
--- a/quickstep/src/com/android/quickstep/TaskAnimationManager.java
+++ b/quickstep/src/com/android/quickstep/TaskAnimationManager.java
@@ -71,6 +71,12 @@
             @Override
             public void onRecentsAnimationStart(RecentsAnimationController controller,
                     RecentsAnimationTargets targets) {
+                if (mCallbacks == null) {
+                    // It's possible for the recents animation to have finished and be cleaned up
+                    // by the time we process the start callback, and in that case, just we can skip
+                    // handling this call entirely
+                    return;
+                }
                 mController = controller;
                 mTargets = targets;
             }