GraphQL Mutation has a problem about variables

Question

I have a .NET Core Web API and I am using in-browser GraphiQL for testing my API. Here is my query and the error I got.

My Schema class:

public class DeveloperSchema : Schema
    {
        public DeveloperSchema(IDependencyResolver resolver) : base(resolver)
        {
            Query = resolver.Resolve<DevelopersQuery>();
            Mutation = resolver.Resolve<DevelopersMutation>();
        }
    }

My Mutation class:

public class DevelopersMutation : ObjectGraphType
    {
        public DevelopersMutation(DevelopersContext _developersContext)
        {
            Field<DeveloperType>(
                "CreateDeveloper",
                arguments: new QueryArguments(
                  new QueryArgument<NonNullGraphType<DeveloperInputType>> { Name = "developer" }
                ),
                resolve: context =>
                {
                    var developer = context.GetArgument<Developer>("developer");
                    return _developersContext.AddDeveloper(developer);
                });
        }
    }

InputType for developer:

public class DeveloperInputType : InputObjectGraphType
    {
        public DeveloperInputType()
        {
            Name = "DeveloperInput";
            Field<NonNullGraphType<IdGraphType>>("Id");
            Field<StringGraphType>("Name");
        }
    }

Developer model:

public class Developer
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

I am using GraphQL 2.4.0, GraphiQL 3.4.0, .NET Core 3.1

I did not share my controller and QueryType structure since my query calls are working fine. I am having trouble with mutation calls.