Fix NPE from accessing pausing tasks

- It's possible for mPausingTasks to be unset between
  startRecentsTransition() and startAnimation(), and in these cases
  canceling with a screenshot can not take the snapshots as we have no
  leashes of the tasks in the animation itself.  For now, just cancel
  the transition without the snapshots.

Fixes: 286874048
Test: Just adding an NPE check
Change-Id: Ie1315d8f6451cfb9b2ee2782c7d89db3cbb36aac

libs/WindowManager/Shell/src/com/android/wm/shell/recents/RecentsTransitionHandler.java

@@ -313,7 +313,7 @@ public class RecentsTransitionHandler implements Transitions.TransitionHandler {
         private Pair<int[], TaskSnapshot[]> getSnapshotsForPausingTasks() {
             int[] taskIds = null;
             TaskSnapshot[] snapshots = null;
-            if (mPausingTasks.size() > 0) {
+            if (mPausingTasks != null && mPausingTasks.size() > 0) {
                 taskIds = new int[mPausingTasks.size()];
                 snapshots = new TaskSnapshot[mPausingTasks.size()];
                 try {