BPM - To validate Plant in RMA Credit Invoice

Hi all,

I have been trying to add a BPM to validate an stop whenever there is a
blank Plant during the raising of an RMA Credit Invoice in RMA

I have tried the BO.Method ARInvoice.Update or RMAProc.RMACreditAdd
and also tried all means like Pre-Processing and Post-Processing but to
no success.

The Plant field in RMA Credit Invoice form seems to be "untouchable" by
any BPM Derivatives I have tried.

Please help.