when user selects anyother value rather than nil i want to make form readonly so i used this but it is not working if anyone have idea please help me here is my code                 <sheet readonly="state !='nil'">

               <field name="state"/>

python code: state = fields.Selection([

        ('nil', 'Nil'),

        ('open', 'open'),

        ('in_progress', 'In Progress'),

        ('resolved', 'Resolved'),

        ('closed', 'Closed')

    ], string="Status", default="nil")