7.0.0: Cannot load notebook with saved widget output

Hello,

We have a problem with the new version of ipywidgets (or more likely with the widgetsnbextension), where a notebook with a saved widget output cannot be loaded.

When loading such a notebook the following error message is thrown in the browser (Firefox):

Notebook failed to load

The error was:
TypeError: Jupyter.notebook.kernel is null

See the error console for details.

It occurs in widgetsnbextension’s render function. It seems as if the kernel was not yet ready when the widget manager property is accessed.

We have the following library versions for Python2 and Python3 respectively:

root@de79f18a1f6a:~# pip2 show ipython notebook nbformat ipywidgets widgetsnbextension traitlets | grep Version -B1
Name: ipython
Version: 5.4.1
--
Name: notebook
Version: 5.0.0
--
Name: nbformat
Version: 4.3.0
--
Name: ipywidgets
Version: 7.0.0
--
Name: widgetsnbextension
Version: 3.0.0
--
Name: traitlets
Version: 4.3.2


root@de79f18a1f6a:~# pip3 show ipython notebook nbformat ipywidgets widgetsnbextension traitlets | grep Version -B1
Name: ipython
Version: 6.1.0
--
Name: notebook
Version: 5.0.0
--
Name: nbformat
Version: 4.3.0
--
Name: ipywidgets
Version: 7.0.0
--
Name: widgetsnbextension
Version: 3.0.0
--
Name: traitlets
Version: 4.3.2

We use Python2 kernels for our notebooks, and here is an example notebook with which we can reproduce the problem: https://gist.github.com/ak0ska/efd644222cbbe936c822f941c3e708fa

If I clear all output, save and reload the notebook, there is no error message. If I execute all cells, save the notebook and reload, I get the error message. The example works without problems with ipywdgets 6 and widgetsnbextension 2.

Any ideas what might be wrong?

Thank you, Akos

[michaelaye]

The update to 3.0.2 also solved this error for me (using Chrome on OSX 10.12.6):

[jasongrout]

@Caelin1234 - what versions of the notebook/lab and ipywidgets do you have installed? What widgets are you using? Can you post a simplified example just using core ipywidgets (sliders, text boxes, etc.) illustrating the issue?