Dúvida JInternalFrame

Programação Java
#1

Pessoal estou com o seguinte problema:
meu JInternalframe nao fica com a mesma aparecia do frame principal, estou postando uma imagem abaixo pra ficar mais explícito o meu problema. desde já agradeço toda ajuda e atenção dispensada

o jinternalframe também nao permite ser redimensionado nem minimizado apesar de estrar true na chamada do metodo. ele sempre fica do tamanho maximo do jframe e eu tambem nao usei o setMaximum.
toda ajuda é bem vinda.

#2

Jah descobri que isso e coisa do look end feel, eu alterei pro look and feel do sistema, mas quando vai executar ele retorna um erro ao montar a janela, o erro segue abaixo

E:\Java\Projetos>java HelloWorldFrame
Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: javax.swing.plaf.ColorUIResourc
e cannot be cast to java.util.List
        at javax.swing.plaf.metal.MetalUtils.drawGradient(Unknown Source)
        at javax.swing.plaf.metal.MetalInternalFrameTitlePane.paintComponent(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JLayeredPane.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintToOffscreen(Unknown Source)
        at javax.swing.BufferStrategyPaintManager.paint(Unknown Source)
        at javax.swing.RepaintManager.paint(Unknown Source)
        at javax.swing.JComponent._paintImmediately(Unknown Source)
        at javax.swing.JComponent.paintImmediately(Unknown Source)
        at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
        at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
        at javax.swing.RepaintManager.seqPaintDirtyRegions(Unknown Source)
        at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(Unknown Source)
        at java.awt.event.InvocationEvent.dispatch(Unknown Source)
        at java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.run(Unknown Source)
#3