Removed class constraints.

Removed class constraints. This allows the use of structs.

Author: mackysoft

Assets/MackySoft/MackySoft.SerializeReferenceExtensions/Editor/SubclassSelectorDrawer.cs

@@ -63,7 +63,6 @@ TypePopupCache GetTypePopup (SerializedProperty property) {
 				Type baseType = property.GetManagedReferenceFieldType();
 				var popup = new AdvancedTypePopup(
 					TypeCache.GetTypesDerivedFrom(baseType).Where(p =>
-						p.IsClass &&
 						(p.IsPublic || p.IsNestedPublic) &&
 						!p.IsAbstract &&
 						!p.IsGenericType &&