Spigot crashes as dragon is taking off from portal when a specific player build exists. The player guild includes a number of blocks in the air above the exit portal (y115 ish). Spigot does not crash every time the dragon takes off from the exit portal, but it has crashed every fight.

The crashes have all been observed when the player is in survival mode.
The crashes have all been observed when the end crystals on the pillars have been broken.
Issue was first observed with git-Spigot-3fb9445-2b6c9f4 (earlier not tested).
There is no crash when the build above the exit portal is removed.
There is no crash when there is a block at 0,115,0 (though the dragon then lands at y115 rather than on the exit portal)

Crash report below as comment so it is collapsible.
Trimmed world that works well with no plugins provided. (Original build was glass/ice/etc and while it still crashes with 0 plugins, it gets difficult to use with the dragon removing blocks. Provided world has had a lot of blocks removed, and a lot of blocks replaced with endstone. It still exhibits the same crashing behavior.)