Virtual functions that return nullable objects fail to compile

[hugopl]

Gio::ListModel has a virtual function called get_item that returns an GObject::Object or nil, however if the code returns nil it fails to compile:

There was a problem expanding macro '_register_get_item_vfunc'

Code in macro 'method_added'

 5 | _register_get_item_vfunc(get_item)
     ^
Called macro defined in lib/gi-crystal/src/auto/gio-2.0/list_model.cr:72:13

 72 | private macro _register_get_item_vfunc(impl_method_name)

Which expanded to:

 > 10 | 
 > 11 |         __temp_70 = __temp_69.as(self).get_item(position)
 > 12 |         __temp_70.to_unsafe
                          ^--------
Error: undefined method 'to_unsafe' for Nil (compile-time type is (Doc | Nil))

In my code, Doc was a class that inherits GObject::Object.