i already installed G4P in my processing, when i push "GUI BUILDER", this error came out

Exception in thread "AWT-EventQueue-0" java.lang.NoSuchMethodError: processing.app.Editor.getBase()Lprocessing/app/Base;
    at g4p.tool.G4PTool.run(Unknown Source)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

how to solve this?
i used processing 1.2.1

thanks