Thanks for contacting support.
We have tested the scenario in our environment using Aspose.Diagram for .NET 18.10 and following code snippet. We were unable to notice the exception that you have mentioned. However, we did notice that output SVG was not correct and logged an issue as DIAGRAMNET-51577 in our issue tracking system.
var d = new Aspose.Diagram.Diagram(dataDir + "source.vsd");
d.Save(dataDir + "source.svg", Aspose.Diagram.SaveFileFormat.SVG);
source.svg.zip (3.5 KB)
We will further investigate above logged issue in details and keep you posted with the status of its correction. Please be patient and spare us little time.
We are sorry for the inconvenience.