handle ValueError

[neon-ninja]

Hi,

This PR fixes the following exception:

ValueError                                Traceback (most recent call last)
<ipython-input-7-705f095156ba> in <module>
----> 1 get_ipython().run_cell_magic('execute_cell', ' EB910D7B09F2443081D464E1B36C9392', 'x = 4\n')

/usr/local/lib/python3.8/dist-packages/IPython/core/interactiveshell.py in run_cell_magic(self, magic_name, line, cell)
   2401             with self.builtin_trap:
   2402                 args = (magic_arg_s, cell)
-> 2403                 result = fn(*args, **kwargs)
   2404             return result
   2405 

~/git/nodebook/nodebook/ipython/nodebookext.py in execute_cell(line, cell)
     70     print(line.lstrip().split(' '))
     71     assert NODEBOOK_STATE['nodebook'] is not None, "Nodebook not initialized, please use %nodebook {nodebook_name}"
---> 72     cell_id, parent_id = line.lstrip().split(' ')
     73 
     74     # make sure cell exists and is in the right position

ValueError: not enough values to unpack (expected 2, got 1)

This problem appears to have been caused by ipython rstriping the %%execute_cell line magic - if the trailing space were there, parent_id would be ''. Note this problem only occurs with the first cell in the notebook, as it's the only cell without a parent.

[kzielnicki]

Hi! Sorry to not notice your PR, nodebook hasn't been in active use or development for some time. I'm happy to merge your fix though, thanks for submitting it!