Apologize that I mislead you - while I testing the Grid code that you provide on our side I didn't notice that you are using nested object. Basically the PopUp template of the Grid
uses the EditorForModel
MVC helper to generate the template, which indeed doesn't support complex properties. That why in current case you should implement custom PopUp editor template.
Also please note that our CodeLibrary already contains such demo which you can use as a baseline:
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