fix possible npe

java.lang.NullPointerException
null
java.awt.geom.RectangularShape.setFrameFromDiagonal(Unknown Source)
edu.uci.ics.jung.visualization.control.PickingGraphMousePlugin.mouseDragged(PickingGraphMousePlugin.java:295)
edu.uci.ics.jung.visualization.control.PluggableGraphMouse.mouseDragged(PluggableGraphMouse.java:133)
java.awt.AWTEventMulticaster.mouseDragged(Unknown Source)