I want to add more property for every shape
So I tried to create a cusom class that inherited from Nshape
but I failed because the Nshape has no new construction and is created by factory
How could I implement this functionality

Still sending the same error

Failed to save to file. Exception was: There was an error generating the XML document.

I wonder, I put a breakpoint in the load xml and save xml in the custom property class and the compiler not going to this line