Cannot convert from GLib.List<Gtk.Window>' to GLib.List<weak Gtk.Window>

[scottw-finao]

Under ubuntu 20.04, it will not compile with the current distribution libs. It's failing on the step:

glib-compile-resources gresource.xml --generate-source --target=gresource.c &&
valac
-o ../gtk-hypermd
-g
--enable-checking
--gresources gresource.xml
--pkg gio-2.0
--pkg granite
--pkg gtk+-3.0
--pkg json-glib-1.0
--pkg libsoup-2.4
--pkg posix
--pkg webkit2gtk-4.0
--Xcc=-Wno-deprecated-declarations
*.vala gresource.c &&
rm -f gresource.c

failing specifically with the error:

Application.vala:139.37-139.43: error: Assignment: Cannot convert from `GLib.List<Gtk.Window>' to `GLib.List<weak Gtk.Window>?'
unowned List windows = get_windows();

[scottw-finao]

I tried removing the 'weak' and it seemed to compile but did have 9 warnings.