'Object reference not set to an instance of an object.' throwing when i am trying to send request with Mediatr

Question

I am trying to send some http request by using Mediatr, but it returning me Nullreference Exception

[HttpPost("Authorize")]
public async void Authorize( MakeOrAuthorizeUserDto makeOrAuthorizeUserDto)
{
    await _mediator.Send(new UserAuthorize.Request(makeOrAuthorizeUserDto));
}

In this method i am adding Mediatr

public static class ServiceExtensions
{
    public static void AddApplicationLayer(this IServiceCollection services)
    {
        services.AddMediatR(cfg => cfg.RegisterServicesFromAssemblies(Assembly.GetExecutingAssembly()));
    }
}

And in program.cs i am adding this method

builder.Services.AddApplicationLayer();

I am using Mediatr 12.1.1 version, i also tried older versions(with extensions library of course), but it didn't works as well

I am trying to connect to DB by senfing this request, but Meediatr throwing me exception as i described earlier