message: Prevent an error when closing a pane which is next to a viewer tab
diff: @@ -812,10 +812,15 @@ export class MainWebUi extends ThemeableElementBase implements keybindingmanager
  this._splitLayout.update();
  this._refreshSplitLayout();
  + if (focusInfo == null) {
  + const tabWidget = this._splitLayout.getTabWidgetByTabContent(tabContentElement);
  + focusInfo = {tabWidget, tabContent: tabContentElement};
  + }
  +
  if (focusInfo.tabWidget != null) {
  focusInfo.tabWidget.focus();
  if (focusInfo.tabContent != null) {
  - if (focusInfo.tabContent instanceof EtTerminal || focusInfo.tabContent instanceof EmptyPaneMenu) {
  + if (focusInfo.tabContent instanceof EtTerminal || focusInfo.tabContent instanceof EmptyPaneMenu || focusInfo.tabContent instanceof EtViewerTab) {
  focusInfo.tabContent.focus();
  }
  }
  