Fix NPE when deciding whether to defer task stack animation.

Bug: 78084664
Test: Just adding an NPE check
Change-Id: I1f07488c5b6e4ad4d1c528e35b7f6b9b00d1fb1b

services/core/java/com/android/server/wm/TaskStack.java

@@ -1721,6 +1721,9 @@ public class TaskStack extends WindowContainer<Task> implements
             return true;
         }
         final Task homeTask = homeStack.getTopChild();
+        if (homeTask == null) {
+            return true;
+        }
         final AppWindowToken homeApp = homeTask.getTopVisibleAppToken();
         if (!homeTask.isVisible() || homeApp == null) {
             return true;