Polygon.Points has wrong nullability attributes

[Ralalu]

Describe the bug
Adding points to a Shape (Polygon, Polyline,...) throw NullReferenceException:

To Reproduce

            Polyline polyLine = new Polyline()
            {
                Stroke = new SolidColorBrush(Colors.Green),
                StrokeThickness = 2.0
            };

            Avalonia.Point a = new Avalonia.Point(80, 90);
            Avalonia.Point b = new Avalonia.Point(250, 250);
            Avalonia.Point c = new Avalonia.Point(240, 220);

            //NullReferenceException occurs here:
            polyLine.Points.Add(a);
            polyLine.Points.Add(b);
            polyLine.Points.Add(c);

            canvas.Children.Add(polyLine);

Expected behavior

My current workaround:
(Or do I have to do it like this?)

            Polyline polyLine = new Polyline()
            {
                Stroke = new SolidColorBrush(Colors.Green),
                StrokeThickness = 2.0
            };

            Avalonia.Point a = new Avalonia.Point(80, 90);
            Avalonia.Point b = new Avalonia.Point(250, 250);
            Avalonia.Point c = new Avalonia.Point(240, 220);

            IList<Avalonia.Point> points = new List<Avalonia.Point>();
            points.Add(a);
            points.Add(b);
            points.Add(c);

            polyLine.Points = points;

            canvas.Children.Add(polyLine);

Desktop (please complete the following information):

• OS: Windows 11
• Visual Studio 2022
• Version: Avalonia 11-Pre6

[maxkatz6]

Yes, it is by design as it seems from the API. Since it's a styled property.
But it would be less confusing, if nullable attributes were set properly.
I will this issue open until somebody fixes nullable attributes on this property.

[3steve3]

I want to add the adding in the second example is a little verbose when you can initialize a list with those points like this:

IList<Avalonia.Point> points = new List<Avalonia.Point>
{
    a,
    b,
    c
};

[lhsrebel72]

@maxkatz6 would this be an appropriate fix for this or am I misunderstanding?
public Polygon() { SetCurrentValue(PointsProperty, new Points()); }

changed from
public Polygon() { Points = new Points(); }

which was throwing a warning saying it should use setcurrentvalue when setting it's own styled or attached properties

[maxkatz6]

@lhsrebel72 no, it shouldn't be changed. See #10370 (comment)

As for this issue, it was resolved before, and that code doesn't throw anymore.