w3c widgets workaround

src/domain/UBGraphicsWidgetItem.cpp

@@ -563,6 +563,9 @@ void UBGraphicsWidgetItem::injectInlineJavaScript()
 
 void UBGraphicsWidgetItem::paint( QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
 {
+    if(!scene() || !scene()->renderingContext())
+        return;
+
     if (scene()->renderingContext() != UBGraphicsScene::Screen)
     {
         painter->drawPixmap(0, 0, snapshot());