Ticking entity (ender dragon) / IllegalStateException: Removing entity while ticking

While checking the DragonPhases for bugpost MC-147152, I summoned 4 dragons, after that I crashed with "Ticking entity".

Upon re-entering the worldsave, I crashed again:

Exception stopping the server java.lang.IllegalStateException: Removing entity while ticking!
at ve.h(SourceFile:958) at ve.e(SourceFile:975) at xo.c(SourceFile:336) at vw.a(SourceFile:1011)
at ja.l(SourceFile:401) at agb.f(SourceFile:75) at vw.b(SourceFile:281) at xo.s(SourceFile:756) 
at net.minecraft.server.MinecraftServer.l(SourceFile:554) at dwo.l(SourceFile:251) 
at net.minecraft.server.MinecraftServer.run(SourceFile:663) at java.lang.Thread.run(Unknown Source)