How to create a list of users based on the roles in ASP.NET Core 6

I have created a model for my users which inherits from IdentityUser.
Here is my model:

 public class ApplicationUser:IdentityUser
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string FatherName { get; set; }
        public string TelephoneNO { get; set; }
        public string Address { get; set; }
        public JobTitle JobTitle { get; set; }
        public Degree Degree { get; set; }
        
        //Relationships
        public string? UserId { get; set; }
        public virtual ApplicationUser User { get; set; }
        public int? ClassroomId { get; set; }
        public Classroom Classroom { get; set; }

        public virtual ICollection<Homework> Homeworks { get; set; }
        public virtual ICollection<UserMajorCourse> UsersMajorsCourses { get; set; }
        public virtual ICollection<UserHomework> UsersHomeworks { get; set; }
        }
    }
}

I want to list the users based on their roles.
Here is my role class:

 public static class UserRoles
    {
        public const string Admin = "Admin";
        public const string Teacher = "Teacher";
        public const string Student = "Student";

    }

I tried using the following method, but I received a null exception error.

var allStudents = _userManager.GetUsersInRoleAsync("Student");

How can I get the list of users based on their roles?

You can get all users in the user manager and then sort through them based on their roles. Similar to something like this.

public async List<ApplicationUser> GetUsersInRole(string role)
{
    var allUsers = await _userManager.Users.ToListAsync();
    var usersInRole = new List<ApplicationUser>();
    foreach (ApplicationUser user in allUsers)
    {
         var roles = await _userManager.GetRolesAsync(user);
         if (roles.Contains(role))
              usersInRole.Add(user);
    }

    return usersInRole;
}