Fix another cause of NRE's.

~~This one stops the game from locking up when you try to control a kerbal.~~

EDIT: Nope, although if it's possible for flightglobals.activevessel to be null we might want to include this one anyway - I'll dig more into the kerbal lockup later.

Unfortunately I managed to reproduce the cant-select-anything-in-the-space-center bug with these changes.