Testing ASP.NET Core API controller with InMemory

I am trying to test an ASP.NET Core API controller using an InMemory database, instead of a SQL server. I am also using NUnit to write my tests. In my SetUp-method, I create some data and add to my InMemory context, which seems to work fine, but when I try to retrieve the data using my controller, I get null values.

This is part of my controller:

public class PeopleController : ControllerBase
    {
        private ApplicationDbContext _context;
        private IMapper _mapper;
        public PeopleController(ApplicationDbContext context, IMapper mapper)
        {
            _context = context;
            _mapper = mapper;
        }

        // GET /api/people
        [HttpGet]
        public IActionResult GetPeople()
        {
            return Ok(_context.People.ToList().Select(_mapper.Map<Person, PersonDto>));
        }
    }

And below is part of my test class. When I debug the test, the two Person objects I add to _context have been added correctly, but when I call _controller.GetPeople(), the two objects do not show up, but I get null back. The method works fine "live", using my SQL Server.

[TestFixture]
    class PeopleControllerTests
    {
        private ApplicationDbContext _context;
        private IMapper _mapper;
        private PeopleController _controller;

        [SetUp]
        public void SetUp()
        {
            _mapper = GenerateConcreteInstance();

            var options = new DbContextOptionsBuilder<ApplicationDbContext>().UseInMemoryDatabase(databaseName: "TestDb").Options;
            _context = new ApplicationDbContext(options);

            _context.People.Add(new Person()
            { Id = 1, Firstname = "XXXXX", Lastname = "XXXXXXX", Email = "XXXX@XXXX.com", City = "XXXXX", DateCreated = DateTime.Now });
            _context.People.Add(new Person()
            { Id = 2, Firstname = "YYYYY", Lastname = "YYYYYYY", Email = "YYYY@YYYY.com", City = "YYYYY", DateCreated = DateTime.Now });

            _controller = new PeopleController(_context, _mapper);
        }

        [Test]
        public void GetAll_WhenCalled_ReturnPeopleInDb()
        {
            var result = _controller.GetPeople();

            var okObjectResult = result as OkObjectResult;
            var content = okObjectResult.Value as IEnumerable<Person>;
            Assert.IsNotNull(content);

        }

        private IMapper GenerateConcreteInstance()
        {
            var config = new AutoMapper.MapperConfiguration(c =>
            {
                c.AddProfile(new ApplicationProfile());
            });

            return config.CreateMapper();
        }
    }

Would really appreciate any help, as I am new to ASP.NET Core and testing in general!