Could not open file. TypeError: can not read property 'cells' of null

Hi,

I have issues open an. `.ipynb` . I followed installation instructions and determined path to `jupyter`. But when I hit ' alt-cmd-o' via `advanced-open-file` in order to define `.ipynb` I'll get `Could not open file. TypeError: can not read property 'cells' of null`. Opening a new file and saving it as `.ipynb` doesnt initiate the package as well. `Hydrogen`for example works well so I assumed the set up is correct. Appreciate any help!

`OSX El Capitan
10.11.4
Atom 1.6.2`

Start by reproducing the failure in Atom 1.6.2 on OSX El Capitan: use advanced-open-file with alt-cmd-o to open an existing .ipynb, then try creating and saving a new one. The work is done when both notebook-opening paths avoid the reported TypeError; the issue names no source file or test to begin with.

Written by the indexing model from the issue text.