Theia attempts to open notebooks as editors (and fails)

[msujew]

Bug Description:

After #13217, I've noticed that when editing a notebook file for the first time (thereby setting its state to dirty), Theia will attempt to open the file as a normal monaco editor, which then fails:

Steps to Reproduce:

1. Open a notebook widget
2. Edit a cell
3. The error should appear in the console

[msujew]

This doesn't seem to happen anymore. Likely due to improvements in Monaco & Notebook handling.