Click under project tree control (#33)

When I click on the empty area under tree control in the projects tab, I always caught an exception (stacktrace below):

System.NullReferenceException: Object reference not set to an instance of an object.

at SoftwareIdeasModeler.Controls.FastTree.FastTree.FastTree_MouseUp(Object sender, MouseEventArgs e)

at System.Windows.Forms.Control.OnMouseUp(MouseEventArgs e)

at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)

at System.Windows.Forms.Control.WndProc(Message& m)

at System.Windows.Forms.ScrollableControl.WndProc(Message& m)

at System.Windows.Forms.ContainerControl.WndProc(Message& m)

at System.Windows.Forms.UserControl.WndProc(Message& m)

at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)

at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)

at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)