RDCMan v3.21 crashing

Getting the following error

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

at RDCMan.Nodes.Server.Dock()

at RdcMan.ServerForm.OnClosed(EventArgs e)

at System.Windows.Forms.Form.WmClose(Message& m)

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

at RdcMan.RdcBaseForm.WndProc(Message& m)

at System.Windows.Forms.NativeWindow.Callback(HWND hWnd, UInt32 msg, WPARAM wparam, LPARAM lparam)

The error seems to happen when I have an undocked session, I restart the remote machine on the the undocked window, and it stays open, when I try to reconnect, on the same undocked window, it opens another docked session, I can login to the new session, and undock it, so now I have two undocked windows, the old one (disconnected) and the new one. I try to close the old one and I get the error shown below, (details above). If I click Continue, nothing happens, if I hit Quit, it closes RDCMan, as stated in the window.