Msgbox in trigger

[#1]

I hope someone can help me with this issue.

I have a trigger in the document, where its function is more than selecting a specific value of
a multiple selection table shows a msgbox, but my problem is that deleting the selection again shows me the msgbox
and should not show it, as I can do to prevent display to clear the selection

Trigger is at field level and is only on the value conciderado "When".

regards,
Alberto