Sending support messages throws error as bot if user is not set

[kailan]

https://sentry.io/syosset/syossethscom/issues/471090253/

Mongoid::Errors::InvalidFind: 
message:
  Calling Document.find with nil is invalid.
summary:
  Document.find expects the parameters to be 1 or more ids, and will return a single document if 1 id is provided, otherwise an array of documents if multiple ids are provided.
resolution:
  Most likely this is caused by passing parameters directly through to the find, and the parameter either is not present or the key from which it is accessed is incorrect.
  app/controllers/message_threads_controller.rb:22:in `send_message'
    user = User.find(params[:user_id])
...
(66 additional frame(s) were not displayed)

Mongoid::Errors::InvalidFind: 
message:
  Calling Document.find with nil is invalid.
summary:
  Document.find expects the parameters to be 1 or more ids, and will return a single document if 1 id is provided, otherwise an array of documents if multiple ids are provided.
resolution:
  Most likely this is caused by passing parameters directly through to the find, and the parameter either is not present or the key from which it is accessed is incorrect.