stop tracking if we don't have a owner anymore, should help with bug #573

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@22559 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/kits/interface/MenuBar.cpp

@@ -476,7 +476,7 @@ BMenuBar::Track(int32 *action, int32 startIndex, bool showMenu)
 
 	while (true) {
 		bigtime_t snoozeAmount = 40000;
-		bool locked = window->Lock();//WithTimeout(200000)
+		bool locked = (Window() != NULL && window->Lock());//WithTimeout(200000)
 		if (!locked)
 			break;