I agree that tape might be your friend here, but it can also be quite ugly. And of course, it also leaves a sticky residue which might well be hard to remove from the crack in the future. Tape will also usually yellow and become brittle over time, meaning the crack might start spreading again without you realising it. Therefore, if the crack is definitely spreading, and if it were my figure, since the alternative is a destroyed bubble, I would be tempted to try using a
very fine bead of superglue or even Araldite at each end of the crack to halt its progress. Depending on how good your join is, that might be enough to hold the bubble together without the crack needing gluing along its entire length (which would almost certainly look crap).
Of course if you did decide to glue it, naturally if you ever came to sell the card, whilst I imagine it would probably be pretty obvious that it had been repaired, nevertheless you should of course fully declare any restorative work carried out on the card. Apologies if this is teaching granny to suck eggs