Although if the context is an event, you cannot add an exit task (because an event lasts a split second so it would exit after that split second) - so the option doesn't appear. Maybe this is not all events but certainly the ones I've tested.
Usually the exit tasks work better on states.
So if you had a profile...
context > Event > Docked
Task > "Car Mode"
This would need to be changed to
context > state > Docked
Task > "car mode"
Exit Task > "Kill App"
...so at the instant the state Docked ceases to be true, the profile exits, running teh exit task.