Salvadoreno,
Rubini here. I'm miles away from my home in a vacation trip. So only sporadic connections...
But ,yes, all the time you dock at any base (milk cow base or not) than not your home base you will received a "NULL" as a destination grid. This is a stock SH3 problem. But in fact this is not a real problem. Just go ahead acomplish your mission (previous grid destination) and so on. All your goodies (medals, renown, etc will be there when you return to your home base. In the milk cow base (or like I said, in any other base which is not your home base) you have only the option to manage your crew. So give the crew medals, crew promotions and crew assignments there. Do this every time you dock at a milk cow base. Later when you docked at your home base again don't forget to adjust the days in base back to 28 using SH3Cmdr.
Enjoy!
Rubini.