Adding your own custom API

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Hi, I want to begin adding a custom API to scripts I make so I made a seperate project in IntelliJ, created a .jar file and added it as a library to my main project. The problem is when I try to start my script after doing this it instantly stops. When I remove the .jar it works again, so this is what's causing my script to stop. Anyone else had this problem? Am I doing this incorrectly?