Add null check for invalid previously appeared target

* In ag/23680185, a null check was removed that was
checking if individual mLastAppearedTaskTargets were null;
we check if the array overall is null, but individual
elements can also be null, ex 3P launcher

Bug: 289609734
Test: Repro steps from b/289609734 don't cause crash
Flag: none
Change-Id: Iddfde6d9ac2b708380b70b5fb6301b629506619c
diff --git a/quickstep/src/com/android/quickstep/TaskAnimationManager.java b/quickstep/src/com/android/quickstep/TaskAnimationManager.java
index 739f10e..c0684d7 100644
--- a/quickstep/src/com/android/quickstep/TaskAnimationManager.java
+++ b/quickstep/src/com/android/quickstep/TaskAnimationManager.java
@@ -206,7 +206,8 @@
                     if (mLastAppearedTaskTargets != null) {
                         for (RemoteAnimationTarget lastTarget : mLastAppearedTaskTargets) {
                             for (RemoteAnimationTarget appearedTarget : appearedTaskTargets) {
-                                if (appearedTarget.taskId != lastTarget.taskId) {
+                                if (lastTarget != null &&
+                                        appearedTarget.taskId != lastTarget.taskId) {
                                     mController.removeTaskTarget(lastTarget.taskId);
                                 }
                             }