I was trying to make two many2many fields inside configuration settings of website. I could make both fields fields but I was not able to save the multiple entered values.  This is the error that occured

 File "/home/RobertD'Souza/Desktop/odoo-13.0/odoo/models.py", line 1688, in _add_missing_default_values     if self._fields[name].type == 'many2many' and value and isinstance(value[0], int):
KeyError: 'web_products'

And this is the code I used

class ResConfigSettings(models.TransientModel):
    _inherit = 'res.config.settings'
    web_product = fields.Many2many('product.template', string="Website Products")
    web_category = fields.Many2many('product.public.category', string="Website Categories")

    def set_values(self):
        res = super(ResConfigSettings, self).set_values()
        self.env['ir.config_parameter'].set_param('website_product_visibility.web_product', self.web_product)
        self.env['ir.config_parameter'].set_param('website_product_visibility.web_category', self.web_category)
        return res

    @api.model
    def get_values(self):
        res = super(ResConfigSettings, self).get_values()
        ICPsudo = self.env['ir.config_parameter'].sudo()
        web_product = ICPsudo.get_param('website_product_visibility.web_product')
        web_category = ICPsudo.get_param('website_product_visibility.web_category')
        res.update(
            web_products=web_product,
            web_categories=web_category
        )
        return res