title: Action Cable error Could not execute command from...RuntimeError - Unable to find subscription with identifier
body: <p>I am running into an error when make a call to perform upstream to an Action Cable channel, 'UsersChannel'. This is the complete error message:</p>
  
  <p>Could not execute command from ({"command"=>"message", "identifier"=>"{\"channel\":\"UsersChannel\"}", "data"=>"{\"id\":\"1\",\"action\":\"show\"}"}) [RuntimeError - Unable to find subscription with identifier: {"channel":"UsersChannel"}]: /Users/robskrob/.rvm/gems/ruby-2.4.1/gems/actioncable-5.1.4/lib/action_cable/connection/subscriptions.rb:76:in <code>find' | /Users/robskrob/.rvm/gems/ruby-2.4.1/gems/actioncable-5.1.4/lib/action_cable/connection/subscriptions.rb:53:in</code>perform_action' | /Users/robskrob/.rvm/gems/ruby-2.4.1/gems/actioncable-5.1.4/lib/action_cable/connection/subscriptions.rb:17:in <code>execute_command' | /Users/robskrob/.rvm/gems/ruby-2.4.1/gems/actioncable-5.1.4/lib/action_cable/connection/base.rb:85:in</code>dispatch_websocket_message' | /Users/robskrob/.rvm/gems/ruby-2.4.1/gems/actioncable-5.1.4/lib/action_cable/server/worker.rb:58:in `block in invoke'</p>
  
  <p>This is very bizarre because in the <a href="https://github.com/robskrob/skillcoop/blob/feature/signup-user/app/javascript/packs/modules/user.js" rel="nofollow noreferrer">same file</a> I successfully use the subscribed channel to perform upstream to create a user (<code>usersChannel.perform('create', {...})</code>). Why am I seeing the above error when I make a different call on the same subscriber that I used to create a user? It was able to find the <code>UsersChannel</code> subscription previously when calling <code>UsersChannel#create</code>. Why is it throwing this error when calling <code>UsersChannel#show</code> with <code>usersChannel.perform('show', {...})</code>on the front end?</p>