Saving integration model gets error

[beyondpie]

Hi Hanqing,

Thanks for the great package! When I use the integration module, I find it will report error when I plan to save it using SeuratIntegration.save with parameter save_adata=True.

TypeError: Cannot setitem on a Categorical with a new category (nan), set the categories first.

Do you know how to fix it? BTW, I am now working on adopting the SeuratIntegration to SnapATAC2. I find the module is written pretty clearly!

Thanks! Songpeng