UIService: quit more gracefully

If the UIService never initialized, and then an AppQuitEvent fires,
there is no need to proactively discover the UIs and save their
locations, since clearly nothing changed with the UI.

Author: ctrueden

src/main/java/org/scijava/ui/DefaultUIService.java

@@ -436,7 +436,10 @@ public void run() {
 	}
 
 	@EventHandler
-	protected void onEvent(@SuppressWarnings("unused") final AppQuitEvent event) {
+	protected synchronized void onEvent(
+		@SuppressWarnings("unused") final AppQuitEvent event)
+	{
+		if (!initialized) return;
 		for (final UserInterface ui : getVisibleUIs()) {
 			ui.saveLocation();
 		}