message: Return undefined from `getAndUpdateModeHandler` if document is closed
  I think this should protect us from disposed editors. It certainly seems like a reasonable safety check, if nothing else.
diff: @@ -32,7 +32,7 @@ export async function getAndUpdateModeHandler(
  forceSyncAndUpdate = false
  ): Promise<ModeHandler | undefined> {
  const activeTextEditor = vscode.window.activeTextEditor;
  - if (activeTextEditor === undefined) {
  + if (activeTextEditor === undefined || activeTextEditor.document.isClosed) {
  return undefined;
  }
  