Fixed random issue in DynamicGridUtils(?)

inabox.wpf/DynamicGrid/DynamicGridUtils.cs

@@ -304,7 +304,7 @@ public static class DynamicGridUtils
                                 }
                                 else
                                 {
-                                    subtypes = _alleltypes.Where(x => x.GetInterfaces().Any(i => i.GenericTypeArguments.Last().Equals(gentype)));
+                                    subtypes = _alleltypes.Where(x => x.GetInterfaces().Any(i => (i.GenericTypeArguments.LastOrDefault()?.Equals(gentype) == true)));
 
                                     if (subtypes.Any())
                                     {