code extensions spring release platformupdate 8

I have the next method

 public static CustVendExchAdj construct(SysModule _module)
    {
        CustVendExchAdj custVendExchAdj;

        if (_module == SysModule::Cust)
        {
            custVendExchAdj = new CustExchAdj();
        }
        else
        {
            custVendExchAdj = new VendExchAdj();
        }

        return custVendExchAdj;
    }

Now I want to extend it, with a post event,

[ExtensionOf(classStr(CustVendExchAdj))]
final static public class MyCustVendExchAdj_Extension
{

    /// <summary>
    ///
    /// </summary>
    /// <param name="args"></param>
    [PostHandlerFor(classStr(CustVendExchAdj), staticMethodStr(CustVendExchAdj, construct))]
    public static void CustVendExchAdj_Post_construct(XppPrePostArgs args)
    {
       
        SysModule typeModel = args.getArg('_module');
        if(typeModel == SysModule::Cust)

But I get object not reference exeptions, could it be that the CustVendExchAdj is an abstract base class?

the funny thing is my debugger see the value correct.