Data retrieval in Blazor app returns null-array

I'm currently struggling creating my first Blazor project that is supposed to be a small CRUD application. I tried to follow some tutorials but most are outdated pre-relase tutorials and there are many differences that confuse me.

my problem is that I don't yet understand some of the basic concepts (especially the 'services'). When i start the app, I get the following error:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Temporary local of type 'UserManagement.Models.Users[]> was null.

The error occurs at the line "@foreach (var user in users)" in the Index.razor. I would understand that there was no data retrieved. However, There is data in my Users table. So i really don't know that the error could be here.

What we have so far is:

Index.razor

@page "/"
@using Models
@using Data
@inject UserService us

<form method="post">
    <input asp-page="/Create" type="button" value="Neuen Benutzer erstellen" />
</form>

<form method="post">
    <table class="table">
        <thead>
            <tr>
                <th>ID</th>
                <th>UserID</th>
                <th>Name</th>
                <th>Supervisor</th>
                <th>Ersteller</th>
                <th>Erstelldatum</th>
                <th>Optionen</th>
            </tr>
        </thead>
        <tbody>
            @foreach (var user in users)
            {
                <tr>
                    <td>@user.PkId</td>
                    <td>@user.UserId</td>
                    <td>@user.FirstName @user.LastName</td>
                    <td>@user.Supervisor</td>
                    <td>@user.CreationUser</td>
                    <td>@user.CreationDate</td>
                    <td>
                        <a>Delete Account</a><br />
                        <a asp-page="/Edit"> Edit Account</a>
                    </td>
                </tr>
            }
        </tbody>

    </table>
</form>

@code {

    Users[] users;

    protected override async Task OnInitializedAsync()
    {
        users = await us.GetUsersAsync();
    }
}
}

UserService.cs

using Microsoft.EntityFrameworkCore;
using System.Collections.Generic;
using System.Threading.Tasks;
using UserManagement.Models;

namespace UserManagement.Data
{
    public class UserService
    {
        private static  UserManagementContext _context;

        public UserService (UserManagementContext context)
        {
            _context = context;
        }

        public async Task<Users[]> GetUsersAsync()
        {
            Users[] u;
            u = await _context.Users.ToArrayAsync();
            return u;

        }
    }
}

The model: Users.cs

using System;
using System.Collections.Generic;

namespace UserManagement.Models
{
    public partial class Users
    {
        public int PkId { get; set; }
        public int FkJobtitle { get; set; }
        public int FkCostcenter { get; set; }
        public int? FkLocation { get; set; }
        public int? FkDepartment { get; set; }
        public int? FkWorkplace { get; set; }
        public int? FkLanguage { get; set; }
        public string UserId { get; set; }
        public string Salutation { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string PersonalId { get; set; }
        public string Supervisor { get; set; }
        public string Telephone { get; set; }
        public DateTime ValidFrom { get; set; }
        public DateTime CreationDate { get; set; }
        public string CreationUser { get; set; }
        public DateTime? ModificationDate { get; set; }
        public string ModificationUser { get; set; }

        public virtual CostCenter FkCostcenterNavigation { get; set; }
        public virtual Department FkDepartmentNavigation { get; set; }
        public virtual Jobtitle FkJobtitleNavigation { get; set; }
        public virtual Language FkLanguageNavigation { get; set; }
        public virtual Location FkLocationNavigation { get; set; }
    }
}

And of course a scaffolded DbContext (excerpt):

using System;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata;

namespace UserManagement.Models
{
    public partial class UserManagementContext : DbContext
    {
        public UserManagementContext()
        {
        }

        public UserManagementContext(DbContextOptions<UserManagementContext> options)
            : base(options)
        {
        }

        public virtual DbSet<Approver> Approver { get; set; }
        public virtual DbSet<Branche> Branche { get; set; }
        public virtual DbSet<CostCenter> CostCenter { get; set; }
        public virtual DbSet<DealerCompany> DealerCompany { get; set; }
        public virtual DbSet<Department> Department { get; set; }
        public virtual DbSet<Jobtitle> Jobtitle { get; set; }
        public virtual DbSet<Language> Language { get; set; }
        public virtual DbSet<Location> Location { get; set; }
        public virtual DbSet<Request> Request { get; set; }
        public virtual DbSet<RequestTypes> RequestTypes { get; set; }
        public virtual DbSet<SystemRole> SystemRole { get; set; }
        public virtual DbSet<SystemType> SystemType { get; set; }
        public virtual DbSet<Systems> Systems { get; set; }
        public virtual DbSet<Users> Users { get; set; }
        public virtual DbSet<Workplace> Workplace { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            if (!optionsBuilder.IsConfigured)
.....

In my startup, I had to load the UserService by using services.AddTransient instead of AddSingleton, because the latter would run into following error:

System.AggregateException: 'Some services are not able to be constructed (Error while validating the service descriptor 'ServiceType: UserManagement.Data.UserService Lifetime: Singleton ImplementationType: UserManagement.Data.UserService': Unable to resolve service for type 'UserManagement.Models.UserManagementContext' while attempting to activate 'UserManagement.Data.UserService'.)'