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

<!-- ⚠️⚠️ Do Not Delete This! bug_report_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Also please test using the latest insiders build to make sure your issue has not already been fixed: https://code.visualstudio.com/insiders/ --> <!-- Use Help > Report Issue to prefill these. --> - 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](https://github.com/microsoft/vscode-extension-samples/tree/master/custom-editor-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://imgur.com/53j7C1u): https://github.com/hediet/vscode-drawio/issues/61