Add null check in transientlaunchrestoretarget

Some sequences of events result in this being null, so
prevent a crash by checking.

Bug: 210014802
Test: manual
Change-Id: Id1b8b5b23d51a9fb5228e7106e2d2614a6d62676

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

@@ -216,6 +216,7 @@ class Transition extends Binder implements BLASTSyncEngine.TransactionReadyListe
     }
 
     Task getTransientLaunchRestoreTarget(@NonNull WindowContainer container) {
+        if (mTransientLaunches == null) return null;
         for (int i = 0; i < mTransientLaunches.size(); ++i) {
             if (mTransientLaunches.keyAt(i).isDescendantOf(container)) {
                 return mTransientLaunches.valueAt(i);