Compatible for Redmine 5.0.3(x) version.

[sh970901]

1. Add "$LOAD_PATH.unshift "#{File.dirname(FILE)}/lib"" at the top of the init.rb file.
2. Change the "before_filter" in the redmine_zendesk_issue_bridge_controller.rb file to "before_action"
3. If you cannot find RetrieveTicketData in the retrieve_ticket_data.rb file, modify it as follows:

  module RetrieveTicketData
    def self.included(base) # :nodoc:
      base.extend(ClassMethods)
      base.class_eval do
        unloadable
      end
    end
  end

 module ClassMethods
    def zendesk_settings
      Setting.plugin_redmine_zendesk_issue_bridge || {}
    end
  end
end

Issue.send(:include, RedmineZendeskIssueBridge::RetrieveTicketData)

4. If the retrieved value you received is [], please try querying with the token. When retrieving the token, make sure to append "/token" to the username.

   #           config.token = "user_zendesk_token"
   #           config.username = "username/token"`

5. Now it should be ready for normal usage.