Jingle GUIs are made with the IntelliJ IDEA form designer, if you intend on changing GUI portions of the code, IntelliJ IDEA must be configured in a certain way to ensure the GUI form works properly:
Settings
->Build, Execution, Deployment
->Build Tools
->Gradle
->Build and run using: IntelliJ Idea
Settings
->Editor
->GUI Designer
->Generate GUI into: Java source code