Custom editors cannot be opened when a single-folder workspace has Been Extended To A Multi-Folder Workspace

• VSCode Version: 1.45.1 / Insiders 2020-05-26T15:51:05.752Z
• OS Version: Windows 10 / Mac

Steps to Reproduce:

1. Check out Cat Customs Sample
2. Install the extension
3. Copy the exampleFiles folder to exampleFiles2
4. Open a new VS Code Instance with exampleFiles as root folder
5. (Custom Editor works as intended)
6. Add exampleFiles2 to the workspace.
7. Open example.cscratch in exampleFile2 folder.
8. It crashes.

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

You can see this happening with my draw.io extension too: https://github.com/hediet/vscode-drawio/issues/61