Exception handlers not called when exception is thrown when parsing variables

[pelletier197]

Describe the bug When using request variables in GraphQL, the behaviour is really different than without variables. My model has a query using variables, and when passing null in a non-nullable field, my custom exception handler is not called. Even worse, the backend actually returns 500, instead of the normal 200.

I haven't yet found a way to handle this error correctly.

To Reproduce

This is a minimal example:

# schema.graphqls

extend type Query {
  groups(filters: GroupFilters): [Group!]!
}

input GroupFilters {
  ids: [String!] # Notice the non-nullable string array element
}

type Group {
   id: String!
   name: String!
}

Calling the model (using the playground) with the following query

# Query
query GetEntity($filters: GroupFilters) {
  groups(filters: $filters) {
    id,
    name
  }
}

and the following variables

{"filters": {"ids": [null]}}

End up in a 500, and my custom exception handler not being called.

Expected behavior The same request, but not using any variables (so passing the filters directly) returns the following response

{
  "errors": [
    {
      "message": "Validation error of type WrongType: argument 'filters.ids[0]' with value 'NullValue{}' must not be null @ 'groups'",
      "locations": []
    }
  ],
  "extensions": {},
  "data": null
}

Screenshots (My actual model is a bit more complex, but here's what I have) Using filters directly: it works

*Using variables: returns 500