asp.net core testing controller with IStringLocalizer

I have controller with localization

public class HomeController : Controller
{
    private readonly IStringLocalizer<HomeController> _localizer;

    public HomeController(IStringLocalizer<HomeController> localizer)
    {
        _localizer = localizer;
    }

    [HttpPost]
    public IActionResult SetLanguage(string culture, string returnUrl)
    {
        Response.Cookies.Append(
            CookieRequestCultureProvider.DefaultCookieName,
            CookieRequestCultureProvider.MakeCookieValue(new RequestCulture(culture)),
            new CookieOptions { Expires = DateTimeOffset.UtcNow.AddYears(1) }
        );

        return LocalRedirect(returnUrl);
    }

    public IActionResult Index()
    {
        ViewData["MyTitle"] = _localizer["Hello my dear friend!"];

        return View("Index");
    }
}

and I added xUnit project for testing and writed next code

public class HomeControllerTest
{
    private readonly IStringLocalizer<HomeController> _localizer;
    private HomeController _controller;
    private ViewResult _result;

    public HomeControllerTest()
    {
        _controller = new HomeController(_localizer);
        _result = _controller.Index() as ViewResult;
    }

    [Fact]
    public void IndexViewDataMessage()
    {
        // Assert
        Assert.Equal("Hello my dear friend!", _result?.ViewData["Message"]);
    }

    [Fact]
    public void IndexViewResultNotNull()
    {
        // Assert
        Assert.NotNull(_result);
    }

    [Fact]
    public void IndexViewNameEqualIndex()
    {
        // Assert
        Assert.Equal("Index", _result?.ViewName);
    }
}

When I running all tests, they returns false with exception:
"Message: System.NullReferenceException : Object reference not set to an instance of an object."

When you double-click on a method in the StackTrace cursor appears on the line
ViewData["MyTitle"] = _localizer["Hello my dear friend!"];

I think this is due to IStringLocalizer. How to fix it? May be somebody knows what is the reason?