.dspproj file will not open "Error reading graph from a file"

I am running SigmaStudio 4.1 on my Windows10 machine.  I created a .dspproj project file and saved it but when I attempt to re-open the project I get the error below and the file will not open.

Error reading graph from a file

Value cannot be null.

Parameter name: item

Please help I put a lot of work into creating this project.