Hello Théo Da Silva, 

You are getting this issue because "crm_user_id" is not defined inside the windows action and by default in Odoo you can't pass value in domain inside a XML file for windows action.

And your requirement is to show leads from Partner on button click

So instead of creating a computed field inside the partner you can achieve the same using button method. 

def action_open_crm_lead(self):
        self.ensure_one()
        userObjs = self.env['res.users'].search([('partner_id', '=', self.id)], limit=1)
        user_id = userObjs.id if userObjs else False
        domain = [('user_id', '=', user_id)] if user_id else []
        return {
            'name': 'CRM LEAD',
            'domain': domain,
            'res_model': 'crm.lead',
            'type': 'ir.actions.act_window',
            'view_mode': 'tree,form',
            'view_id': False,
        }

Feel free to ask your queries

Thanks

Anisha Bahukhandi