Can't be in the context of nil. [, <!DOCTYPE html><html lang="en"></html> ]

Hi @gbisheimer 

I'm using this gem for activeadmin 2.1.0. After install, when I restarted server, I got following error:

```
ArgumentError in Admin::Users#index
Can't be in the context of nil. [, <!DOCTYPE html><html lang="en"></html>
]

```

Can you please let me know if we can use this gem with latest activeadmin 2.1.0. If yes, how can  I fix this issue?

Thanks