Thanks, it worked!
In the 'Add Task to Things' workflow I added an output -> Run Script and set the language to /usr/bin/osascript
Then I entered the following code:
delay 15
tell application "Things" to quit
I did not even need to force Things to sync as it syncs by itself within 10sec (but you could probably set it to more if you need, maybe for an other app?)
Anyway, it workss now, I'm happy, and thanks for your help!