FYI - I found this same exception can happen with enderpearls too. The single-argument constructor for it was also removed. I had it happen on my server when someone was porting to the Nether, although not sure exact repro steps.
It would be clear though if a Mojang developer looked at the thrown enderpearl class. It would be the exact same fix for fireball and wither skull (adding a constructor that accepts only World parameter).
MC-64486
FYI: That call stack includes a Skeleton entity (class name "afm" in snapshot 31a).
Maybe the HashSet in the EntityTracker class needs to be a ConcurrentHashSet.
Looks like the constructor with just one World parameter was removed in the latest release for both the WitherSkull and Fireball classes.
FYI - I found this same exception can happen with enderpearls too. The single-argument constructor for it was also removed. I had it happen on my server when someone was porting to the Nether, although not sure exact repro steps.
It would be clear though if a Mojang developer looked at the thrown enderpearl class. It would be the exact same fix for fireball and wither skull (adding a constructor that accepts only World parameter).