Assert that `get_adjustment` doesn't return `NULL`
Assert that `get_adjustment` doesn't return `NULL`

`Range`, `ScaleButton` and `SpinButton` never have `NULL` for `adjustment`.

EPashkin/rust-gnome-gtk,daa84/gtk,Susurrus/gtk,EPashkin/rust-gnome-gtk,Susurrus/gtk,gkoz/rust-gnome-gtk,gkoz/rust-gnome-gtk,gkoz/gtk-rs-gtk,gkoz/gtk-rs-gtk,daa84/gtk,rust-gnome/gtk,rust-gnome/gtk