Hi Rakesh,
I faced with same trouble. I resolve it by using RFx - Document Lifecycle Event - "Validated" instead of "Post Phase".
Then to have "Validated" script like run Phase Change add condition. For example fo our case we need to prevent RFx from beeing awarded in case RFx field was null.
if(doc.getCurrentPhase().equals(new RfxSchedPhaseEnumType(6)) && field.get() == null ) {
throw new ApplicationException(session);
}