Added missing null-check while rendering groups

RGB.NET.Core/RGBSurface.cs

@@ -93,7 +93,7 @@ namespace RGB.NET.Core
             IList<Led> leds = ledGroup.GetLeds().ToList();
             IBrush brush = ledGroup.Brush;
 
-            if (!brush.IsEnabled) return;
+            if ((brush == null) || !brush.IsEnabled) return;
 
             try
             {