I get this "NULL" sector to patrol when:
I add/remove a mod (I am at my homebase atm)
I refuel and rearm at a friendly port or ship (e.g. Bessel at Vigo ... "das ist in Spanien"
)
What does it mean?
Simply spoken: you do not get a sector to patrol so you can go hunting freely but also can't get the 500+200 renown from a patrol.
To avoid this (and for all the other specials) I use Sh3 Commander and set the Patrol Sector to my favourite: BF17