Softlocked out of a quest

I am unable to complete the Treasure of Utzaal quest because I died right when i killed the boss. As a result I could not pick up the boss item and it is now forever out of my reach because I can no longer enter the arena.

This feels like something that should have been accounted for given the nature of this boss.

https://gyazo.com/39d941ce6476cc3ed3584f5e10eeb2d2