Bukkit/CraftBukkit — pull request #577 by TheEliteFour (2011-12-07 23:28:57+00:00)
Title: Teach EnderDragons to not attempt to destroy Air.

java.lang.NullPointerException
at org.bukkit.craftbukkit.CraftWorld.explodeBlock(CraftWorld.java:909)
at net.minecraft.server.EntityEnderDragon.a(EntityEnderDragon.java:498)
at net.minecraft.server.EntityEnderDragon.d(EntityEnderDragon.java:330)
at net.minecraft.server.EntityLiving.w_(EntityLiving.java:297)
at net.minecraft.server.World.entityJoinedWorld(World.java:1253)
at net.minecraft.server.WorldServer.entityJoinedWorld(WorldServer.java:107)

Is caused when you try to find the natural drops for Air. Air cannot be destroyed so should not throw an explosion event anyway.