Hello Rajesh,
I was not able to reproduce the issue that you have mentioned here. I have tested this with the version V4.1.0.62 by creating a small sample and have attached the same here.
Kindly look at the attached sample''s CustomDiagram.cs file. Please let me know whether this helps you.
If you still run into any problem, could you please modify the sample to reproduce the null reference exception issue. So that it will be easier for us to find out the cause for the problem.
Thanks for your interest in Syncfusion products.
Regards,
Meera.
CustomDiagram.zip