QmlDesigner.Naviagor: crash fix for slides in model

For some reason gradient stops end up in the navigator model.
This should be fixed for 3.1.

Change-Id: Ic447a6907409e4e9776cfd164a49bb6194cc4cab
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

src/plugins/qmldesigner/components/navigator/navigatortreemodel.cpp

@@ -298,7 +298,9 @@ void NavigatorTreeModel::updateItemRowOrder(const NodeListProperty &listProperty
     } else {
         parentIdItem = itemRow.idItem->parent();
     }
-    Q_ASSERT(parentIdItem);
+
+    if (!parentIdItem) //### Items without a parent should not exist
+        return;
 
     if (currentRow != newRow) {
         QList<QStandardItem*> items = parentIdItem->takeRow(currentRow);