Exception when drawing Filter's histogram

[Mathieu Bastian]

Way to reproduce:
1) Open Graph file
2) Create new Degree Range filter
3) Select the filter
An exception occurs sometimes

Stack trace:
java.lang.NullPointerException
        at org.gephi.ui.filters.plugin.JQuickHistogram$JQuickHistogramPanel.drawHisto(JQuickHistogram.java:200)
        at org.gephi.ui.filters.plugin.JQuickHistogram$JQuickHistogramPanel.paintComponent(JQuickHistogram.java:184)
        at javax.swing.JComponent.paint(JComponent.java:1029)
        at javax.swing.JComponent.paintChildren(JComponent.java:864)
        at javax.swing.JComponent.paint(JComponent.java:1038)
        at javax.swing.JComponent.paintChildren(JComponent.java:864)
        at javax.swing.JComponent.paint(JComponent.java:1038)
        at javax.swing.JComponent.paintChildren(JComponent.java:864)
        at javax.swing.JComponent.paint(JComponent.java:1038)
        at javax.swing.JComponent.paintChildren(JComponent.java:864)
        at javax.swing.JComponent.paint(JComponent.java:1038)
        at javax.swing.JComponent.paintChildren(JComponent.java:864)
        at javax.swing.JComponent.paint(JComponent.java:1038)
        at javax.swing.JComponent.paintToOffscreen(JComponent.java:5124)
        at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1475)
        at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1406)
        at javax.swing.RepaintManager.paint(RepaintManager.java:1220)
        at javax.swing.JComponent._paintImmediately(JComponent.java:5072)
        at javax.swing.JComponent.paintImmediately(JComponent.java:4882)
        at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:803)
        at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:714)
        at javax.swing.RepaintManager.seqPaintDirtyRegions(RepaintManager.java:694)
        at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(SystemEventQueueUtilities.java:128)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:125)
        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)