Use ICore::dialogParent for message boxes without parent so far

Change-Id: I2ee35796b2c041e6ea2ae6bd677e10167451d1b8
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/qmldesigner/components/formeditor/dragtool.cpp

@@ -35,6 +35,8 @@
 #include <metainfo.h>
 #include <rewritingexception.h>
 
+#include <coreplugin/icore.h>
+
 #include <QGraphicsSceneMouseEvent>
 #include <QDebug>
 #include <QMimeData>
@@ -275,7 +277,7 @@ void DragTool::commitTransaction()
     try {
         m_rewriterTransaction.commit();
     } catch (RewritingException &e) {
-        QMessageBox::warning(0, "Error", e.description());
+        QMessageBox::warning(Core::ICore::dialogParent(), "Error", e.description());
     }
 }