SCXML editor: fix crash on pressing copy/cut

QtCreator used to crash when copy or cut button were pressed without
having selected an item.

Task-number: QTCREATORBUG-17637
Change-Id: Iceaea7a9f826ed16c4caee9fc28b4c14eb304c62
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/scxmleditor/plugin_interface/graphicsscene.cpp

@@ -216,6 +216,9 @@ void GraphicsScene::removeSelectedItems()
 
 void GraphicsScene::copy()
 {
+    if (!m_document->currentTag())
+        return;
+
     QPointF minPos;
     QVector<ScxmlTag*> tags;
     if (m_document->currentTag()->tagType() == Scxml) {