Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.66.0
• OS Version: Windows 10 Pro

Steps to Reproduce:

1. Run custom-editor-sample from vscode-extension-samples repo
2. Open example.cscratch
3. Rename file in explorer view to Example.cscratch

Fails with error:

Unable to open 'Example.cscratch': Assertion Failed: argument is undefined or null.

Rename succeeds if the new filename is totally different, but fails if it differs only in case. See also:

jjuback/gc-excelviewer#188
microsoft/vscode-extension-samples#582