Upgraded Xamarin to Maui Project Throws NullReference Exception While Executing HttpClient.GetAsync

I recently upgraded my Xamarin project to Maui and now I get a nullrefference during a GetRequest in my HttpClient but can't find where.

My HttpClient is filled in correctly and I call it like shown below

but I get this Exception:

This is the base of the controller which makes the HttpClient:

    public BaseControllerService()
    {
        //GetUrl();
        AuthorisationRetry = Policy
            .HandleResult<HttpResponseMessage>(resp => resp.StatusCode == HttpStatusCode.Unauthorized)
            .RetryAsync(1, async (_, retryCount) =>
            {
                var answer = await CallRefreshToken();
                if (answer is {IsSuccess: true}) return;
                await _caching.DeleteTokensAsync();
                await Shell.Current.Navigation.PopToRootAsync();
                await Shell.Current.GoToAsync("//LoginPage");
            });
        var test = AppSettings.Url.Value;
        _cookieContainer = new CookieContainer();
        var handler = new HttpClientHandler {CookieContainer = _cookieContainer};
            _httpClient = new HttpClient(handler)
            {
                BaseAddress = new Uri(AppSettings.Url.Value ?? throw new NullReferenceException("AppSettings Url is null"))
            };
        SetHeaders();
    }

    protected async void SetHeaders()
    {
        _httpClient.DefaultRequestHeaders.Clear();

        var temp = await _caching.GetUserTokenAsync();
        if (!string.IsNullOrEmpty(temp)) _httpClient.DefaultRequestHeaders.Add("Authorization", $"Bearer {temp}");
    }

so apearently I have to do DI differently in Maui and thus during runtime _caching in the SetHeaders() was null

edit:
at this moment I have implemented DI in a extension method using

MauiAppBuilder.Services.AddSingleton<T>();

and getting it in the constructor
instead of using

Dependencyservice.Get<T>()

in the class