java.lang.IllegalStateException

java.lang.IllegalStateException: Yaml is wrong configured: '' is not a valid language. See configuration comments for details.
	at de.leonhard.xcore.data.yaml.language.LanguageManager.getLanguage(LanguageManager.java:30)
	at de.leonhard.xcore.XCoreBungee.onStart(XCoreBungee.java:76)
	at de.leonhard.xcore.sapphire.Sapphire$Bungee.onEnable(Sapphire.java:176)
	at net.md_5.bungee.api.plugin.PluginManager.enablePlugins(PluginManager.java:247)
	at net.md_5.bungee.BungeeCord.start(BungeeCord.java:279)
	at net.md_5.bungee.BungeeCordLauncher.main(BungeeCordLauncher.java:62)
	at net.md_5.bungee.Bootstrap.main(Bootstrap.java:15)