Is anyone stumble over situation when in module you start put any @api decorator and it crashes odoo?

I have module with states. I'm trying to signed button function to change state. But when i writing any @api decorator
and restart server, reload web page, it pushes me to form of choosing database or shows continious messages about connection lost and retrieve.

When i comment @api it works fine up to moment of button click.

Here is my simple model:

from odoo import models, fields, api

class la_shipping(models.Model):
    _name = 'tralalal'
    _description = 'Tralala'

@api.multi
def button_done(self):
    for rec in self:
        rec.write({'state', 'done'})

state = fields.Selection([('sent', 'Sent'),('done','Done')],
                                    required =True, default='sent')

<header>
    <button name="button_done" string="Done" states="sent" class="oe_highlight" type="object"></button>
    <button name="button_sent" string="Sent" states="sent,done" class="oe_highlight" type="object"></button>
    <field name="state" widget="statusbar"></field>
</header>