QmlDesigner: Fix crash

If the view is not attached we should not access a nullptr.

Task-number: QDS-4382
Change-Id: I885f2b1b810d4ffd7b87749ca50e06cbdac3a7e2
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/designercore/model/abstractview.cpp

@@ -699,6 +699,7 @@ void AbstractView::setEnabled(bool b)
 
 QList<ModelNode> AbstractView::allModelNodes() const
 {
+    QTC_ASSERT(model(), return {});
     return toModelNodeList(model()->d->allNodes());
 }