Remove polygonItems from their Stroke when added to a StrokesGroup

src/domain/UBGraphicsPolygonItem.cpp

@@ -114,6 +114,9 @@ UBGraphicsPolygonItem::~UBGraphicsPolygonItem()
 
 void UBGraphicsPolygonItem::setStrokesGroup(UBGraphicsStrokesGroup *group)
 {
+    if (mStroke)
+        mStroke->remove(this);
+
     mpGroup = group;
 }