testDeleteItemDuringSubmenuDisplayInterval() {
    mockClock = new MockClock(true);

    const submenu = new SubMenu('submenu');
    submenu.addItem(new MenuItem('submenu item 1'));
    menu.addItem(submenu);

    // Trigger mouseover, and remove item before showSubMenu can be called.
    /** @suppress {checkTypes} suppression added to enable type checking */
    const e = new GoogEvent();
    submenu.handleMouseOver(e);
    menu.removeItem(submenu);
    mockClock.tick(SubMenu.MENU_DELAY_MS);
    // (No JS error should occur.)
  }
If you remove a submenu in the interval between when a mouseover event
is fired on it, and showSubMenu() is called, showSubMenu causes a null
value to be dereferenced. This test validates that the fix for this works.
(See bug 1823144).
@suppress {checkTypes} suppression added to enable type checking