Error uploading private app: undefined method `join' for nil:NilClass

Hi,

I'm working on a private Support app using the ZAF framework. When I go to upload the app within Zendesk I get this error. I've tried running `zcli apps:validate` but it says there are no validation errors. 

For some more context: the app is based on the provided react scaffold, and runs in the ticket_sidebar location. As far as I can tell, the manifest and other required files match what is described in the docs.