hdy_list_box_separator_header() has wrong signature in Vala
public static void list_box_separator_header (Gtk.ListBoxRow row, Gtk.ListBoxRow? before, void* unused_user_data);
../../../../../../../../../Projects/sidebar-example/src/window.vala:32.26-32.54: error: Argument 1: Cannot convert from `void Hdy.list_box_separator_header (Gtk.ListBoxRow, Gtk.ListBoxRow?, void*)' to `void Gtk.ListBoxUpdateHeaderFunc? (Gtk.ListBoxRow, Gtk.ListBoxRow?)'
list.set_header_func (Hdy.list_box_separator_header);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
That void* unused_user_data
shouldn't be there.