error: field name must not be null`.

with secs as (
    select p.page_id, p.domain, s.section_id as sid, s.title as title
    from pages p
    left join sections s on p.page_id = s.page_id
    where p.domain = 'bar.com'
),
txt as (
    select
        sid,
        json_agg(
            json_build_object(
                'Pos', pos,
                'Text', content
            )
            order by pos asc
        ) as txts
    from texts
    join secs on sid = section_id
    group by sid
),
img as (
    select
        sid,
        json_agg(
            json_build_object(
                'Pos', pos,
                'Image', image
            )
            order by pos asc
        ) as imgs
    from images
    join secs on sid = section_id
    group by sid
)
select
    json_build_object(
        'ID', s.page_id,
        'Domain', domain,
        'Sections', json_object_agg ( -- Error occurs here
            s.title,
            json_build_object(
                'ID', s.sid,
                'Texts', t.txts,
                'Images', i.imgs
            )
            order by s.sid asc
        )
    )
from secs s
left join txt t on s.sid = t.sid
left join img i on s.sid = i.sid
group by s.page_id, domain;

create table pages (
    page_id serial primary key,
    domain text unique not null
);

create table sections (
    section_id serial primary key,
    title text not null,
    page_id int references pages
);

create table texts (
    section_id int references sections,
    pos int not null,
    content text not null,
    primary key (section_id, pos)
);

create table images (
    section_id int references sections,
    pos int not null,
    image text not null,
    primary key (section_id, pos)
);

-- spanac.com will have 3 sections with texts and images in each, various amounts
insert into pages (domain) values ('spanac.com');
-- foo.com has 1 empty section
insert into pages (domain) values ('foo.com');
-- bar.com has no sections
insert into pages (domain) values ('bar.com');

-- spanac.com

with s as (
    insert into sections (page_id, title) select page_id, 'first' from pages where domain = 'spanac.com' returning section_id
),
t1 as (
    insert into texts (section_id, pos, content) select section_id, 1, 'spanac one.one' from s
),
t2 as (
    insert into texts (section_id, pos, content) select section_id, 2, 'spanac one.two' from s
),
i1 as (
    insert into images (section_id, pos, image) select section_id, 1, 's11.jpg' from s
)
insert into images (section_id, pos, image) select section_id, 2, 's12.jpg' from s;

with s as (
    insert into sections (page_id, title) select page_id, 'second' from pages where domain = 'spanac.com' returning section_id
),
t1 as (
    insert into texts (section_id, pos, content) select section_id, 1, 'spanac two.one' from s
),
t2 as (
    insert into texts (section_id, pos, content) select section_id, 2, 'spanac two.two' from s
),
i1 as (
    insert into images (section_id, pos, image) select section_id, 1, 's21.jpg' from s
)
insert into images (section_id, pos, image) select section_id, 2, 's22.jpg' from s;

with s as (
    insert into sections (page_id, title) select page_id, 'third' from pages where domain = 'spanac.com' returning section_id
),
t1 as (
    insert into texts (section_id, pos, content) select section_id, 1, 'Spanac three.one' from s
)
insert into images (section_id, pos, image) select section_id, 1, 's31.jpg' from s;

-- foo.com

insert into sections (page_id, title) select page_id, 'empty' from pages where domain = 'foo.com';