If no key file is chosen, but upload is submitted, we error out

If a user submits the upload form, without choosing a file, we error out to:

I, [2017-09-06T14:14:11.495877 #10111]  INFO -- : Started POST "/lists/1/keys" for 127.0.0.1 at 2017-09-06 14:14:11 +0200
I, [2017-09-06T14:14:11.498792 #10111]  INFO -- : Processing by KeysController#create as HTML
I, [2017-09-06T14:14:11.531897 #10111]  INFO -- :   Parameters: {"utf8"=>"✓", "authenticity_token"=>"uhusomevalue", "commit"=>"Upload", "
list_id"=>"1"}
I, [2017-09-06T14:14:11.580515 #10111]  INFO -- : Redirected to 
I, [2017-09-06T14:14:11.589433 #10111]  INFO -- : Completed 500 Internal Server Error in 57ms (ActiveRecord: 0.7ms)
F, [2017-09-06T14:14:11.599107 #10111] FATAL -- : 
NoMethodError (undefined method `index_url' for #<KeysController:0x007f2df0f5f508>
Did you mean?  index):
  app/controllers/keys_controller.rb:24:in `create'