You upgrade libraries in your notebook, even ones that run the notebooks themselves: pip install --upgrade upgrades ALL dependencies of your package, including the notebook package which is the core library here.
Even without the --upgrade switch, your package pulls in a 7.X ipywidgets module, after which you re-enable the notebook extension with the newer version. This is where things break.
If you execute the following code in the default environment you see that the widgets in the default environment are working fine:
from ipywidgets import widgets
from IPython.display import *
from ipywidgets import *
In short, the default environment is fine, however the changes you are making are breaking it. Please write an email to firstname.lastname@example.org so we can fix your environment.