Reading the javadoc on the event tells you: "Note: This event may be called before the underlying Chunk is promoted to ChunkStatus#FULL. You will cause chunk loading deadlocks if you don't delay your world interactions."
As such, you should probably keep a queue of entities added along with the world and position you would like it to spawn at. Then, on the next tick in probably WorldTickEvent, check if the chunk is loaded at the position of spawning and if so spawn the entity. Otherwise, recycle into the queue until next tick.