Custom editor issues: save and undo

<!-- ⚠️⚠️ 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: 1.47.3 - OS Version: Fedora 32 I've been experiencing some intermittent issues related to both save and undo operations on the custom editor that I'm working on. I also managed to reproduce them on the Pawdraw sample. Please refer to the steps below. Notice that, these issues do not occur 100% of the time but, eventually, they will if these steps are stressed enough. **Issue A - Save operation** 1. Open a pawDraw file: fileX 2. Open another pawDraw file: fileY 3. Go back to fileX and make an edit 4. Go to fileY and make an edit 5. Save fileY (ctrl+S) _Notification Error: Failed to save 'fileY.pawDraw': Canceled_ **Issue B - Undo operation** _Notification Info: Could not undo 'Stroke' because there is already an undo or redo operation running._ when executing similar steps as in Issue A, i.e., editing, undoing and going back and forth between 2 files. Please let me know if you need any further information. /cc @mjbvz