SideBarMode: return false in eventFilter if qqw is nullptr

If user presses Alt+Tab in a view that has both `QtWidgets` and `QtQuickWidgets`, and
Alt+Tab is not set as a shortcut, systemsettings will crash.

This seems to be because Alt key activates the quick shortcuts in `QtWidgets`
(the thing that underlines one letter from an element) and the active element
is then not a `QtQuickWidget`, which is then `nullptr`, and crashes systemsettings
when `nullptr` is sent with `sendEvent`

BUG:480006


(cherry picked from commit 4ba7b0b1)