The latter has a way to get EphyTabView from it; if EphyTabView is disposed first, we get a use-after-free.
It doesn't crash atm, but it does in wip/exalm/gtk4-overview as there are AdwTabButton and AdwTabOverview keeping extra refs -> AdwTabView outlifes EphyTabView and things explode when closing a window.
And 2 more fixes, this time for a crash reproducible with plain wip/exalm/gtk4 when closing the last tab in the window.