Saving with custom editor occasionally displays 'Canceled' error message

<!-- ⚠️⚠️ 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. --> - Version: 1.49.1 - Commit: 58bb7b2331731bf72587010e943852e13e6fd3cf - Date: 2020-09-16T23:21:17.533Z - Electron: 9.2.1 - Chrome: 83.0.4103.122 - Node.js: 12.14.1 - V8: 8.3.110.13-electron.0 - OS: Darwin x64 19.6.0 Steps to Reproduce: 1. Install the latest Insider VSIX from here https://pvsc.blob.core.windows.net/extension-builds/ms-python-insiders.vsix 2. Update user settings.json as follows: ```json "python.experiments.optInto": [ "CustomEditorSupport - experiment" ], ``` 3. Create a new notebook 4. Add/Edit few cells, run a few cells & save using `ctrl+s` 5. Eventually you'll get the following error `Failed to save 'xyz.ipynb': Canceled` **Note:** * When we check the raw contents of the ipynb file it does seem to have saved the contents. Not sure why the error appears. * Haven't been able to repro with debug bits.  <!-- Launch with `code --disable-extensions` to check. --> Does this issue occur when all extensions are disabled?: No