MAYA-107386 As a user, on the stage AE template I'd like to connect t…

…o time

MAYA-107350 As a user, on the stage AE template I'd like to see what my default prim is
MAYA-107338 As a user, on the stage AE template I'd like to see what my Root Layer is
MAYA-107273 As a user, on the stage AE template I'd like to toggle Purpose=Default

* Bullet-proof some code that can have null path.

lib/mayaUsd/ufe/StagesSubject.cpp

@@ -316,8 +316,13 @@ void StagesSubject::onStageInvalidate(const MayaUsdProxyStageInvalidateNotice& n
     afterOpen();
 
 #ifdef UFE_V2_FEATURES_AVAILABLE
-    Ufe::SceneItem::Ptr sceneItem = Ufe::Hierarchy::createItem(notice.GetProxyShape().ufePath());
-    Ufe::Scene::instance().notify(Ufe::SubtreeInvalidate(sceneItem));
+    auto p = notice.GetProxyShape().ufePath();
+    if (!p.empty()) {
+        Ufe::SceneItem::Ptr sceneItem = Ufe::Hierarchy::createItem(p);
+        if (sceneItem) {
+            Ufe::Scene::instance().notify(Ufe::SubtreeInvalidate(sceneItem));
+        }
+    }
 #endif
 }