local fake_entity = _entity_from_index(0)
local fake_entity_valid = _make_invalid_entity()
assert_throws(function()
world.insert_children(fake_entity_valid, 1, { fake_entity_valid })
end, "Missing or invalid entity")
local entity = world.spawn()
assert_throws(function()
world.insert_children(entity, 1, { fake_entity })
end, "Are you trying to use an entity in a callback in which it's unavailable?")