Oracle Forms

I tried to catch the name of block ,record and item from which the trigger fire .I wrote the following on the form level :
message(:system.trigger_block||' _ '||:system.trigger_record || ' _ '||:system.trigger_item);
it displayed the block and record names only but nothing for item name .In the other hand it displayed the name of the item with when mouse click.I wonder why it works with this trigger while it didn't with on-error trigger .

Any help would be thinkfull