QmlDesigner: Turn Q_ASSERT into QTC_ASSERT

I had this a couple of times and this does not seem fatal.

Change-Id: Id6d2101a5dc1bdc652e6ca8557c7dd8450399be4
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

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

@@ -43,6 +43,7 @@
 #include <QTime>
 
 #include <utils/algorithm.h>
+#include <utils/qtcassert.h>
 
 namespace QmlDesigner {
 
@@ -85,7 +86,7 @@ void FormEditorScene::resetScene()
 
 FormEditorItem* FormEditorScene::itemForQmlItemNode(const QmlItemNode &qmlItemNode) const
 {
-    Q_ASSERT(qmlItemNode.isValid());
+    QTC_ASSERT(qmlItemNode.isValid(), return 0);
     return m_qmlItemNodeItemHash.value(qmlItemNode);
 }