java error

my little brother is playing runescape constantly and for the past month he is getting a java error on my pc and iwas wondering if anyone can tell me how to fix it. here is what the error looks like

java.lang.NullPointerException: null pData
at sun.awt.windows.WComponentPeer.hide(Native Method)
at java.awt.Component.removeNotify(Unknown Source)
at java.awt.Container.removeNotify(Unknown Source)
at java.awt.Container.removeNotify(Unknown Source)
at java.awt.Container.removeAll(Unknown Source)
at sun.plugin.viewer.frame.IExplorerEmbeddedFrame.windowClosed(Unknown Source)
at java.awt.Window.processWindowEvent(Unknown Source)
at java.awt.Window.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(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)


also is there a way to block runescape from my pc without installing a program on my pc