Before I paste the bug report format below, here is a simple statement to fix the problem. For 1.13 build, change SKULL_ITEM to PLAYER_HEAD inside https://github.com/games647/ChangeSkin/blob/master/bukkit/src/main/java/com/github/games647/changeskin/bukkit/command/SkullCommand.java at line 90.

What behaviour is observed:
Cannot do /skin-skull because SKULL_ITEM was replaced with PLAYER_HEAD in 1.13.

What behaviour is expected:
It should change the skull in my hand to the specified skin id.

Steps/models to reproduce:
1: Load this in spigot 1.13 server.
2: Create a skin id by uploading a skin.
3: Get a player head/skull.
4: Do /skin-skull with skull/player head in hand.
5: Read console for the error, it'll say [ChangeSkin] Task #2165 for ChangeSkin v3.1-SNAPSHOT-bece00f generated an exception
java.lang.NoSuchFieldError: SKULL_ITEM, com.github.games647.changeskin.bukkit.command.SkullCommand.setSkullSkin(SkullCommand.java:90)

Plugin list:
N/A, the error is reproduced without any other minecraft plugins.

Environment description:
N/A, is a problem with the coding/version of this build.

Plugin version or build number:
3.1-SNAPSHOT-bece00f

Error Log:
https://hastebin.com/udesusogol.bash

Configuration:
N/A