Apply Dialog Failed error when assigning Master Page

[Darcy]

I am trying to assign a Master Page to a topic but am receiving an error message: Apply Dialog failed: Object reference not set to an instance of an object.

Does anyone have an idea what might be causing this issue and how to resolve. There are other topics assigned to this Master Page and they seem to be functioning as expected.

Thanks

[JWilliStyle]

I spent over 3 hours trying to resolve this issue. Uninstalling, deleting folders and files, reinstalling, re-importing project, making new master pages, etc. I was using MCF 2019 R2. YMMV, but I finally downloaded MCF 2018 and installed. Now I can apply Master Pages. Not saying this is a definite solution, but hopefully it works for others. I couldn't find anything in the MCF help except for when the same Object Reference error comes up when trying to rename topics.