Fix crash when opening the back/forward/up action menus

The popup menu for back/forward/up on the toolbar crashes KCacheGrind
when KF6 is used.

It seems the KToolBarPopupAction widget changed in KF6, and menu()
is now null() by default, and we should use popupMenu() instead.

See also:
https://api.kde.org/frameworks/kwidgetsaddons/html/classKToolBarPopupAction.html

BUG: 483973
(cherry picked from commit 2dfd6e65)