A bear? Seriously? ...in the middle of a desert and trapped in the a dragon's lair for like what... 100 years? How did he survive in the blistering heat with the confusing desert terrain and only bugs to eat? And then end up in a room with the doors shut. On top of that, it's a POLAR bear! (Or dire bear depending on level) With all due respect to the author, the encounter is a little silly as written -- how does a polar bear get into a desert mirage that only opens once every 100 years?
For potential DM's, my advice is to replace the polar bear with an displacer beast that is an escaped pet from the Gnoll camp and call it a day. Yes, Displacer Beast's natural climate is temperate hills so technically he doesn't belong either, but the Gnolls kept him in a cave to guard mushrooms. He escaped a few days ago, wandered in the desert before coming here to rest and he attacks the PCs on sight because he's starving.
It's a tactically interesting combat (and same CR as the polar bear) which makes a tiny bit more sense than a fur covered, arctic animal in a room with shut doors and no food. At least the Displacer Beast is smart enough to work the doors with his tentacles.