Update method - Value cannot be null. (Parameter 'entity') error 500

I test my app by postman:

statuscode:500

messsage: Value cannot be null. (Parameter 'entity')

details: at Microsoft.EntityFrameworkCore.Utilities.Check.NotNull[T](T value, String parameterName)\r\n at Microsoft.EntityFrameworkCore.DbContext.Entry[TEntity](TEntity entity)\r\n at API.Data.UserRepository.Update(AppUser user) in C:\Users\vu227\DatingApp\API\Data\UserRepository.cs:line 67\r\n at API.Controllers.UsersController.UpdateUser(MemberUpdateDto memberUpdateDto) in C:\Users\vu227\DatingApp\API\Controllers\UsersController.cs:line 51\r\n at lambda_method101(Closure , Object )\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.TaskOfActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()\r\n--- End of stack trace from previous location ---\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Logged|17_1(ResourceInvoker invoker)\r\n at Microsoft.AspNetCore.Routing.EndpointMiddleware.g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)\r\n at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)\r\n at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)\r\n at API.Middleware.ExceptionMiddleware.InvokeAsync(HttpContext context) in C:\Users\vu227\DatingApp\API\Middleware\ExceptionMiddleware.cs:line 29"

AppUser

 using System;
 using System.Collections.Generic;
 using API.Extensions;

 namespace API.Entities
  {
   public class AppUser
    {
    public int Id { get; set; }
    public string UserName { get; set; }
    public  byte[] PasswordHash { get; set; }
    public byte[] PasswordSalt { get; set; }
    public DateTime DateOfBirth { get; set; }
    public string KnownAs { get; set; }
    public DateTime Created { get; set; } = DateTime.Now;
    public DateTime LastActive { get; set; } = DateTime.Now;
    public string  Gender { get; set; }
    public string Introduction { get; set; }
    public string LookingFor { get; set; }
    public string Interests { get; set; }
    public string City { get; set; }
    public string  Country  { get; set; }
    public ICollection<Photo> Photos { get; set; }
}

}

public void Update(AppUser user)
    {
        _context.Entry(user).State = EntityState.Modified;
    }

line 67 in UserRepostory is

_context.Entry(user).State = EntityState.Modified;

Controller Put method looks like:

   [HttpPut]
    public async Task<ActionResult> UpdateUser(MemberUpdateDto memberUpdateDto)
    {
        var claimsIdentity = User.Identity as System.Security.Claims.ClaimsIdentity;
        var username = claimsIdentity.Name;
        var user = await _userRepository.GetUserByUsernameAsync(username);

        _mapper.Map(memberUpdateDto, user);

        _userRepository.Update(user);

        if (await _userRepository.SaveAllAsync()) return NoContent();

        return BadRequest("Failed to update user");
    }

Line 51 is

_userRepository.Update(user);

MemberUpdateDto:

 namespace API.DTOs
  {
   public class MemberUpdateDto
    {
    public string Introduction { get; set; }
    public string LookingFor { get; set; }
    public string Interests { get; set; }
    public string City { get; set; }
    public string Country { get; set; }
    
     }
  }

Mapper:

 public class AutoMapperProfiles : Profile
{
    public AutoMapperProfiles()
    {
        CreateMap<AppUser, MemberDto>()
        .ForMember(dest => dest.PhotoUrl, opt => opt.MapFrom(src => 
                src.Photos.FirstOrDefault(x => x.IsMain).Url))
            .ForMember(dest => dest.Age, opt => opt.MapFrom(src => src.DateOfBirth.CalculateAge()));;
        CreateMap<Photo, PhotoDto>();
        CreateMap<MemberUpdateDto, AppUser>();
    }
}