Hi Lakshmi,
I don't know because they are giving separate event, I do not use Business One Studio. I always wrote my control event in itemEvent. But maybe they separate to maintain the code more cleaner.
I believe that in ClickBeforeEvent does not work, because the correct way for an event is this:
1. Your click in SAP B1 control
2. SAP triggered an event to all add-ons with beforeAction variable equals true(ItemEvent)
3. SAP make your own logic.
4. SAP triggered an event to all add-ons with beforeAction variable equals false(ItemEvent).
I believe when you interrupt the flow in step 2, SAP B1 triggered an error in step 3, because your form doesn't exists anymore.
Regards,
Diego