Avoid a potential NPE.

Test: atest ActivityLifecycleSplitScreenTests
Bug: 142345211
Change-Id: Iad8dcfa8aa2cfdbe5f36d7747d1b7421abe27921

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

@@ -2354,7 +2354,7 @@ class ActivityStarter {
                     mMovedToFront = true;
                 }
 
-                if (launchStack.topTask() == null) {
+                if (launchStack != null && launchStack.topTask() == null) {
                     // The task does not need to be reparented to the launch stack. Remove the
                     // launch stack if there is no activity in it.
                     Slog.w(TAG, "Removing an empty stack: " + launchStack);