Properly remove shadow event listeners to prevent IE8 memory leak (#13129)

Change-Id: I98becf08f56ce35a3ee47650d0d4e204e7d18e73

client/src/com/vaadin/client/ui/VOverlay.java

@@ -241,10 +241,10 @@ public class VOverlay extends PopupPanel implements CloseHandler<PopupPanel> {
 
     private void removeShadowIfPresent() {
         if (isShadowAttached()) {
-            shadow.removeFromParent();
-
             // Remove event listener from the shadow
             unsinkShadowEvents();
+
+            shadow.removeFromParent();
         }
     }