Installer crashes in Java 12 when invoked from CLI

[leijurv]

java -jar ~/Downloads/installer-0.7.1-donator.jar --mode vanilla --no-gui works fine in Java 8, but in Java 12 it results in:

Exception in thread "main" java.util.ConcurrentModificationException
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1134)
	at io.github.ImpactDevelopment.installer.setting.InstallationConfig.getSettingValue(Unknown Source)
	at io.github.ImpactDevelopment.installer.setting.InstallationConfig.execute(Unknown Source)
	at io.github.ImpactDevelopment.installer.Installer.main(Unknown Source)