Jupyter 500 : Internal Server Error – cannot import name ConverterMapping
ImportError: cannot import name ConverterMapping [E 16:24:24.485 NotebookApp] { "Accept-Language": "en-US,en;q=0.5", "Accept-Encoding": "gzip, deflate", "Host": "localhost:8888", "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8", "User-Agent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:70.0) Gecko/20100101 Firefox/70.0", "Connection": "keep-alive", "Referer": "http://localhost:8888/tree", "Cookie": "username-localhost-8888=\"2|1:0|10:1574810660|23: username-localhost-8888|44:ODYwMDdiMDE4MGRjNDkwOWFiMjBlZmI1MThiODY3OTg=| 840976561750da22820e615382453a3a38d20a7e359f75871c3623b1e4d9ebfe\"; _xsrf=2|fe88647d|9b98777c28dce1ff0e17b5f3038d75f6|1574805739", "Upgrade-Insecure-Requests": "1" } [E 16:24:24.485 NotebookApp] 500 GET /notebooks/PytorchTensors.ipynb (127.0.0.1) 189.22ms referer=http://localhost:8888/tree

And this was what worked for me to fix the problem.
conda install --channel=conda-forge nb_conda_kernels