Warning when AP Bill total is zero

Hi i cannot work out what i am missing. I want a warning not a stop message when an AP Bill is created/updated if the Total value is 0. It works but i cannot save the Bill with a zero as the error keeps presenting on save. I have tried Row Inserted but that triggers to early.

  public class APInvoiceEntry_Extension : PXGraphExtension<PX.Objects.AP.APInvoiceEntry>
  {
    #region Event Handlers

    protected void APInvoice_RowPersisted(PXCache cache, PXRowPersistedEventArgs e, PXRowPersisted InvokeBaseHandler)
    {
      if(InvokeBaseHandler != null)
        InvokeBaseHandler(cache, e);
      var row = (APInvoice)e.Row;
                       if (row !=null && row.CuryLineTotal == 0)
            {
                throw new PXSetPropertyException("The document total is zero", PXErrorLevel.Warning);
            }
    }