When I saw the Explorer in Slash GraphQL, I was elated. As I would no longer have to type all the variables structure necessary to make a Query or Mutation work.
However, it failed.
Upon clicking “Extract the current value to GraphQL variable” button next to a key. It did it’s magic, however the mutation failed.
Thanks for this bug report. I’ll add this to our backlog, but I cannot provide a timeline for this bug. In your particular case, you could change the ($id: String) to ($id: String!) and I believe it should have worked.
We currently use GraphiQL explorer for this, so we’ll need to fix this upstream.