fixes #1983

Author: beto-rodriguez

src/skiasharp/LiveChartsCore.SkiaSharp.WinForms/MotionCanvas.cs

@@ -89,7 +89,7 @@ private void SkglControl_PaintSurface(object sender, SKPaintGLSurfaceEventArgs e
             new SkiaSharpDrawingContext(CanvasCore, e.Surface.Canvas, GetBackground()));
 
     private SKColor GetBackground() =>
-        ((IChartView)Parent!)?.BackColor.AsSKColor() ?? SKColor.Empty;
+        (Parent as IChartView)?.BackColor.AsSKColor() ?? SKColor.Empty;
 
     void IRenderMode.InitializeRenderMode(CoreMotionCanvas canvas) =>
         throw new NotImplementedException();