[byamabe]

I had the same behavior when OmniFocus was closed.
I found two non-optimal solutions which involve adding lines after "tell application":
1.
activate

Problem with this is that OmniFocus will always be brought to the front.

2.
launch
delay 1

Problem with this is that the "delay 1" is dependent on the system and is not guaranteed not to crash in the same way. It would be best to ensure that OmniFocus is ready to receive commands, but I don't know enough AppleScript or have the time to do further digging.