Fix VAbstractOrderedLayout widget removal race condition (#18703)

Removing a widget that got moved to another layout occasionally caused
slot to lose track of related widget. As a result the layout never
created a new slot when the same widget is put back in there.

Change-Id: I0d8793324b8a5ac8a06aa2803ac8de22b90b7545

client/src/com/vaadin/client/ui/orderedlayout/VAbstractOrderedLayout.java

      * @return
      */
     public void removeWidget(Widget widget) {
-        Slot slot = widgetToSlot.get(widget);
-        removeSlot(slot);
+        Slot slot = widgetToSlot.remove(widget);
+        if (slot != null) {
+            removeSlot(slot);
+        }
     }
 
     /**
      */
     protected void removeSlot(Slot slot) {
         remove(slot);
-        widgetToSlot.remove(slot.getWidget());
     }
 
     /**