Well, to provide a counterargument, I don't think that even Star Destroyers jump into hyperspace on a whim. Even in the Falcon, I seem to recall them strapping in for the jump and for dropping out. I might be wrong, and I probably am, but I feel like they couldn't just stand about for that sort of stuff. There's a lot of inertia. Either way, I feel like a military vessel, especially one of such scale, would likely have protocols for hyperspace jumps. Clearing the fighter bay prior to a jump doesn't seem impossible. We only ever see the hangars in regular space, it also seems quite possible that it would have mechanical doors that shut for hyperspace journeys. You need hull integrity for that sort of stuff, and having an open hangar with people and smaller ships inside seems like a silly idea.
The 2 most likely options in my mind are that
A. the crew leaves the hangar bay in preparation for a jump, walking through airlock doors. (Like the one that closed after Vader killed Ben). The atmosphere vents, and the forcefield drops. Fighter pilots could survive inside their ships.
or
B. more likely, a large door closes the hangar, keeping the atmosphere sealed in, so the crew can carry on working.
Of course, it's quite possible that the forcefield is just that good.
