deleting issue

hi,
after deleting all the nodes if you press the delete button again it gives you this error and the delete function does not work anymore.
i have tested it on Windows 7 using Google Chrome.

Uncaught TypeError: Cannot read property 'children' of undefined model.js:181
Model.nextSibling model.js:181
commands.cut.apply commands.js:100
Controller.doCommand controller.js:93
Controller.executeCommands controller.js:47
Controller.actions.cut controller.js:135
View.actions.cut view.js:146
(anonymous function) keys.js:58
(anonymous function) view.js:329

by the way, it is very great work but it would be better if you can control creating the nodes and all the other function by buttons too.

Reproduce the issue in Chrome by deleting all nodes and pressing Delete again. Start with model.js:181, then trace the call from commands.js:100 through the listed controller and view handlers. Done means the second delete no longer throws the reported TypeError and the delete function remains usable.

Written by the indexing model from the issue text.