QmlDesigner: Reset item library drag on model detach

Fixes: QDS-7823
Change-Id: If0b758f79dd4a55356d2cd5aefe65fa3e6e321be
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>

src/plugins/qmldesigner/components/itemlibrary/itemlibrarywidget.cpp

@@ -295,8 +295,10 @@ void ItemLibraryWidget::delayedUpdateModel()
 void ItemLibraryWidget::setModel(Model *model)
 {
     m_model = model;
-    if (!model)
+    if (!model) {
+        m_itemToDrag = {};
         return;
+    }
 
     setItemLibraryInfo(model->metaInfo().itemLibraryInfo());