I've a model which inherits a field from another one:

class 2ndclass(models.Model):

    _name = 'mymodel_name'

    _inherits = {'1stclass': 'id_1stclass'}

    id_1stclass = fields.Many2one('1stclass', 'First Class', widget="many2one_list")

When I create a new object, there is a small arrow near 'First Class'. If I click on It, I get this popup error:

Odoo Client Error

TypeError: this.sidebar is undefined

http://localhost:8069/web/js/web.assets_backend/c686f43:3994

Below this popup, there is another one which allows to modify the selected 1stclass id.

How can I solve it?

Thanks!