Bugfix: closing a notebook was generating an error in the completer dispose method.
Bugfix: closing a notebook was generating an error in the completer dispose method.

jupyter/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab