Object reference is required for the non-static field when fetching data from Service

Question

This code is in the code behind:

 protected override async void OnAppearing()
    {
        base.OnAppearing();

        listView.ItemsSource = await Service.DataAsync();
    }

The class that fetches data:

public class Service
{
    public List<TodoItem> TodoList { get; private set; }
    static HttpClient client;

    Service()
    {
        client = new HttpClient();
        client.MaxResponseContentBufferSize = 256000;
    }

    public async Task<List<TodoItem>> DataAsync()
    {

        TodoList = new List<TodoItem>();


        var uri = new Uri(string.Format(Constants.RestUrl, string.Empty));

        try
        {
            var response = await client.GetAsync(uri);
            if (response.IsSuccessStatusCode)
            {
                var content = await response.Content.ReadAsStringAsync();
                TodoList = JsonConvert.DeserializeObject<List<TodoItem>>(content);
                Debug.WriteLine(content);
            }
        }
        catch (Exception ex)
        {
            Debug.WriteLine(@"ERROR {0}", ex.Message);
        }

        return TodoList;
    }
}

When the class and task is static Im able to make this call and populate the listview but not without static. I dont want it to be static. Not duplcate.