#94 creation of substack abort if no selected task

[Benoit GY]

A task must be selected before creating a substask. If not the case, a stack trace appears, see below. I propose to unable the creation of substask if no task selected.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at net.sf.memoranda.ui.treetable.TreeTableModelAdapter.nodeForRow(TreeTableModelAdapter.java:99)
at net.sf.memoranda.ui.treetable.TreeTableModelAdapter.getValueAt(TreeTableModelAdapter.java:103)
at net.sf.memoranda.ui.TaskPanel.addSubTask_actionPerformed(TaskPanel.java:524)
at net.sf.memoranda.ui.TaskPanel$2.actionPerformed(TaskPanel.java:129)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1849)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2169)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:420)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:258)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:231)
at java.awt.Component.processMouseEvent(Component.java:5501)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3135)
at java.awt.Component.processEvent(Component.java:5266)
at java.awt.Container.processEvent(Container.java:1966)
at java.awt.Component.dispatchEventImpl(Component.java:3968)
at java.awt.Container.dispatchEventImpl(Container.java:2024)
at java.awt.Component.dispatchEvent(Component.java:3803)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4212)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3892)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3822)
at java.awt.Container.dispatchEventImpl(Container.java:2010)
at java.awt.Window.dispatchEventImpl(Window.java:1778)
at java.awt.Component.dispatchEvent(Component.java:3803)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:463)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:157)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:149)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)