message: Fix scrollbar update after terminal close
diff: @@ -1375,8 +1375,22 @@ namespace winrt::Microsoft::Terminal::TerminalControl::implementation
  const int viewHeight,
  const int bufferSize)
  {
  + // Since this callback fires from non-UI thread, we might be already
  + // closed/closing.
  + if (_closing.load())
  + {
  + return;
  + }
  +
  // Update our scrollbar
  _scrollBar.Dispatcher().RunAsync(CoreDispatcherPriority::Low, [=]() {
  + // Even if we weren't closed/closing few lines above, we might be
  + // while waiting for this block of code to be dispatched.
  + if (_closing.load())
  + {
  + return;
  + }
  +
  _ScrollbarUpdater(_scrollBar, viewTop, viewHeight, bufferSize);
  });
  