check that email is not an instance of WebUser

this feels like it could be too specific a fix. do you know what code is causing it to be a `WebUser`? Is the intention that that list has always supposed to have either `WebUsers` or email strings and if so can you explain why that's the case (maybe with a comment or doc string somewhere)? if not, i would think the better fix would be to make sure whatever is populating `contact_emails` handles the correct typing.