Whenever DragonBattle.initiateRespawn(); is called even if the ender dragon is killed and there is no other instance of an ender dragon in the world, this will do absolutely nothing. This is a fairly old bug that still has unresolved forums from years ago. The respawn phase is NONE no matter what you do. The ender dragon can still be respawned manually in-game so this is a bug with the method. Also, I have the correct instance of the DragonBattle, I used the get method from the end world and I have done tests such as sending all players in the world a message and verifying that I am getting the correct world instance, this is simply an API bug.

To recreate this bug all you need to do is kill the ender dragon in your end world and include this in your onEnable() method.

Bukkit.getWorld("world_the_end").getEnderDragonBattle().initiateRespawn();