TypeError: Gitlab::RequestForgeryProtection::Controller#index: no implicit conversion of nil into String (TypeError)

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1533435

TypeError: no implicit conversion of true into String (TypeError)

      scanner = StringScanner.new(query_str)
                                  ^^^^^^^^^
  from graphql/language.rb:48:in `initialize'
  from graphql/language.rb:48:in `new'
  from graphql/language.rb:48:in `escape_single_quoted_newlines'
  from app/controllers/graphql_controller.rb:257:in `query'
  from app/controllers/graphql_controller.rb:185:in `any_mutating_query?'
  from app/controllers/graphql_controller.rb:15:in `block in <class:GraphqlController>'

Problem

The application raises a server error when the GraphQL query doesn't have a string type.

Proposal

Explicitly convert queries to String type.