I'm implementing custom cell editor using spreadprocessing to create a template for user to export and edit. After they are done, they import back using the Telerik built-in import function. I have no problem when creating the validation in the cells as I followed your example here:
When I tried to open the export file, I would get an error : "We found a problem with some content in "abcd.xlsx". Do you want us to try to recover as much as we can? ..." After I clicked 'Yes', my custom cell editors disappeared.
The same error occurs when I export from your demo. However, if I create a spreadsheet with validation from scratch, everything works perfectly.