With Retrofit 2, onResponse is called even if there is a failure. You check response.isSuccess(), but if it's false, in your code you just log it. Instead, you could log response.errorBody().string() to see if api server specifies the error, and you should call something like return to exit onResponse callback, as response.body() couldn't be casted as MovieResponse, hence the null exception.
By the way, your code is correct, but if you just start with Retrofit, it would be simpler to use 1.9 version as 2.0 is still a beta version (very stable though, but lack of tutorials).