[Vala] vala compiler crash at GEE lists

[Pavol Klačanský <pavol klacansky com>]

Hi, why this not works? I looked at shotwell code, and it is written
same way in it

** (valac:4554): CRITICAL **: vala_gvariant_module_get_basic_type_info:
assertion `signature != NULL' failed
server.vala:44.9-44.28: error: GVariant serialization of type
`Gee.ArrayList<Feed?>' is not supported
        public Gee.ArrayList<Feed?> get_all_feeds() {
               ^^^^^^^^^^^^^^^^^^^^

** (valac:4554): CRITICAL **: vala_gvariant_module_get_basic_type_info:
assertion `signature != NULL' failed
server.vala:66.9-66.28: error: GVariant serialization of type
`Gee.ArrayList<Item?>' is not supported
        public Gee.ArrayList<Item?> get_all_items(string parent_id = "") {
               ^^^^^^^^^^^^^^^^^^^^

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