GraphQL mutation variables
You probably defined the input name as a non-null string (something like type: new GraphQLNonNull(GraphQLString)
if using js server, or String!
in plain GraphQL).
So your input in the mutation must match, which means it must also be a non-null string. If you change to the following, it should work:
mutation M($name: String!) {
addGroup(name:$name) {
id,
name
}
}
Also if you define a default value as you did, it will be a non-null string.
Finally, you could drop the requirement of being a non-null in the server.