Upstream has released updates that appears to apply and compile correctly.
This update has not been tested by PaperMC and as with ANY update, please do your own testing
CraftBukkit Changes:
5da21f87 SPIGOT-6152: End exit gates in custom ends do not send back to overworld
1ee373fe SPIGOT-6157: Crash when PortalCreateEvent cancelled
e3bbc0d3ec