EF Core - FirstOrDefaultAsync throws Null Reference Exception - Code sample to reproduce the issue.

UsersController.cs

public class UsersController : Controller
{
    private readonly DatabaseContext _databaseContext;
    public UsersController(DatabaseContext databaseContext)
    {
        _databaseContext = databaseContext;
    }

    public async Task<ActionResult<User>> Get(int id)
    {
        var user = await _databaseContext.Users.FirstOrDefaultAsync(x => x.Id == id);
        if (user == null)
        {
            return NotFound();
        }

        return user;
    }
}

UsersControllerTests.cs

public class UsersControllerTests
    {
        [Fact]
        public async Task VerifyGetMethodByIdReturnsNotFoundIfUserDoesntExists()
        {
            using (var dbContext = await GetDatabaseContext())
            {
                var userController = new UsersController(dbContext);

                var user = await userController.Get(100);

                Assert.NotNull(user);
            }
        }

        private async Task<DatabaseContext> GetDatabaseContext()
        {
            var options = new DbContextOptionsBuilder<DatabaseContext>()
                .UseInMemoryDatabase(databaseName: Guid.NewGuid().ToString())
                .Options;
            var databaseContext = new DatabaseContext(options);
            databaseContext.Database.EnsureCreated();
            if (await databaseContext.Users.CountAsync() <= 0)
            {
                for (int i = 1; i <= 10; i++)
                {
                    databaseContext.Users.Add(new User()
                    {
                        Id = i,
                        Email = $"testuser{i}@example.com",
                        IsLocked = false,
                        CreatedBy = "SYSTEM",
                        CreatedDate = DateTime.UtcNow
                    });

                    await databaseContext.SaveChangesAsync();
                }
            }

            return databaseContext;
        }
    }