Fix NPE by making Toast on UI thread

Bug: 271453748
Test: AppTaskTests
Change-Id: I14b3a93951254de75516c343dfd3420b3ed407a8

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

@@ -1729,14 +1729,11 @@ public class ActivityTaskSupervisor implements RecentTasks.Callbacks {
         }
 
         if (ActivitySecurityModelFeatureFlags.shouldShowToast(callingUid)) {
-            Toast toast = Toast.makeText(mService.mContext,
+            UiThread.getHandler().post(() -> Toast.makeText(mService.mContext,
                     (ActivitySecurityModelFeatureFlags.DOC_LINK
-                            + (restrictActivitySwitch
-                            ? "returned home due to "
-                            : "would return home due to ")
-                            + callingLabel),
-                    Toast.LENGTH_LONG);
-            UiThread.getHandler().post(toast::show);
+                            + (restrictActivitySwitch ? " returned home due to "
+                                    : " would return home due to ")
+                            + callingLabel), Toast.LENGTH_LONG).show());
         }
 
         // If the activity switch should be restricted, return home rather than the