ASP.NET service, System.NullReferenceException [duplicate]

This question already has an answer here:

• What is a NullReferenceException, and how do I fix it? 28 answers

I'm working on C# UWP wheather application. I have a problem with ASP.NET web service, when I try to run the service on the browser, I see this error

System.NullReferenceException: Object reference not set to an instance of an object.

Source File: C:\Users\Kaceper\Documents\Visual Studio 2017\Projects\UWPWeatherService\UWPWeatherService\Controllers\HomeController.cs Wiersz: 17

Line 15:         {
Line 16:             // Validation / trimming
Line 17:             var latitude = double.Parse(lat.Substring(0, 5));
Line 18:             var longitude = double.Parse(lon.Substring(0, 5));
Line 19: 

Here is my code:

HomeController.cs

public class HomeController : Controller
{
    public async Task Index(string lat, string lon)
    {
        var latitude = double.Parse(lat.Substring(0, 5));
        var longitude = double.Parse(lon.Substring(0, 5));

    var weather = await Models.OpenWeatherMapProxy.GetWeather(latitude, longitude);

    ViewBag.Name = weather.name;
    ViewBag.Temp = ((int)weather.main.temp).ToString();
    ViewBag.Description = weather.weather[0].description;

    return View();
    }
}

and the URL, which I'm passing in browser:

http://localhost:52035/?lat=35.123456&lon=87.123456

How can I fix this? please help