Fixed a bug causing null reference errors if no graph asset was selected

MaterialGraphProject/Assets/GraphFramework/SerializableGraph/Editor/Drawing/SerializableGraphView.cs

         private void PropagateSelection()
         {
             var graphDataSource = GetPresenter<AbstractGraphDataSource>();
-            if (graphDataSource == null) return;
+            if (graphDataSource == null || graphDataSource.graphAsset == null) return;
 
             var selectedNodeGuids = selection.OfType<NodeDrawer>().Select(x => ((NodeDrawData) x.presenter).node.guid);
             graphDataSource.graphAsset.drawingData.selection = selectedNodeGuids;