Thanks for the help Lizard.
I tried your code and it causes an error when the task is not found instead of returning false.
I think this is the ticket:
Code:
-- Finds a Task
-- _id : tracid of task
on findTask(_id)
tell application "OmniPlan"
tell first document
try
set t to first task whose value of custom data entry "tracid" is _id
on error errmesg number errn
if errn is -1719 then
return false
else
error errmesg number errn
end if
end try
return t
end tell
end tell
end findTask