Fixed the problem of NullPointerException being called.

Release note
・Update to 1.16.1
・Bug that opens the book for those who join for the first time

Warning
・If you are using a previous version, delete it to regenerate the config.

If you update using Patch-1.0.0, delete config.yml! (But be careful if you delete config.yml, the book will also disappear!)
Added OnlyFirstJoin to config.yml.
By setting this to true, only the first player can enter.
If you want to reset the first player to enter, delete players.yml!