QmlDesigner: Add QTC_ASSERT

This might avoid a rare crash on macOS.

Change-Id: I9ea7be826aeaaf571041c0f729bb1fc691e46687
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

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

@@ -318,6 +318,8 @@ void ItemLibraryWidget::setupImportTagWidget()
 
 void ItemLibraryWidget::updateModel()
 {
+    QTC_ASSERT(m_itemLibraryModel, return);
+
     m_itemLibraryModel->update(m_itemLibraryInfo.data(), m_model.data());
     updateImports();
     updateSearch();