I'm trying to migrate an old style wizard to new style. When i try to update the module, openerp raises this error "IntegrityError: null value in column "res_model" violates not-null constraint"

my wizard.xml file contains the form view definition, act_window and menuitem. The code is the following:

    <record id="jasper_wizard_make_template" model="ir.actions.act_window">
        <field name="name">Make XML Template</field>
        <field name="model">jasper_server.wizard.make_template</field>
        <field name="arch" type="xml">
            <form string="Make XML Template" version="7.0">
                <field invisible="1" name="state"/>
        <field name="name" invisible="1"/>
                <group states="init" string="Create Data Template">
        <field name="model" colspan="4" width="300"/>
        <field name="depth"/>
        <field name="model_id"/>
        <field name="indent"/>
                </group>
                <div states="get_file">
                    <h2>Data Template Generated</h2>
                    <p>file: <field name="data" readonly="1" filename="name"/></p>
                </div>
                <footer states="init">
                    <button name="_generate" string="Create" type="object" class="oe_highlight"/> or 
                    <button special="cancel" string="Cancel" type="object" class="oe_link"/>
                </footer>
                <footer states="get_file">
                    <button special="cancel" string="Close" type="object"/>
                </footer>
            </form>
        </field>
    </record>

    <record id="action_jasper_wizard_make_template" model="ir.actions.act_window">
        <field name="name">Make XML Template</field>
        <field name="type">ir.actions.act_window</field>
        <field name="res_model">jasper_server.wizard.make_template</field>
        <field name="view_type">form</field>
        <field name="view_mode">form</field>
        <field name="target">new</field>
    </record>
    <menuitem action="action_jasper_wizard_make_template" id="menu_jasper_wizard_make_template" parent="menu_jasper"/>

jasper_server.wizard.make_template object is the following:

import pooler
import base64

from openerp import tools
from openerp.osv import fields,osv

try:
    from cStringIO import StringIO
except ImportError:
    from StringIO import StringIO

class make_template(osv.osv_memory):
    _name = "jasper_server.wizard.make_template"
    _columns = {
            'name': fields.char('File Name', readonly=True),
            'model': fields.char('Object Model', required=True),
            'depth': fields.integer('Depth', required=True),
            'model_id': fields.many2one('ir.model', 'Object Model', required=True),  # object model in ir.model
            'indent': fields.boolean('Indent', help='Indent the XML output ?'),
            'data': fields.binary('File', readonly=True),
            'state': fields.selection( ( ('init','init'),   # init_form
                                         ('get_file','get_file'),         # save_form
                                       ) ),
    }

    _defaults = { 
        'depth' : lambda *a: 1,
        'state': lambda *a: 'init',
    }

    def _generate(self, cr, uid, data, context):
        """
        Compose the XML
        """
        this = self.browse(cr, uid, ids)[0]
        form = data['form']
        j_obj = pooler.get_pool(cr.dbname).get('jasper.server')
        context['indent'] = this.indent
        buf = StringIO()
        buf.write(j_obj.generator(cr, uid, this.model, this.model_id, this.depth, context=context))

        datas = base64.encodestring(buf.getvalue())
        buf.close()
        filename = 'jasper.xml'
        self.write(cr, uid, ids, {'state': 'get_file',
                                  'data': datas,
                                  'name':filename}, context=context)
        return {
            'type': 'ir.actions.act_window',
            'res_model': 'jasper_server.wizard.make_template',
            'view_mode': 'form',
            'view_type': 'form',
            'res_id': this.id,
            'views': [(False, 'form')],
            'target': 'new',
        }

the error is raisen only when I uncomment the form definition in the wizard.xml, act_window and menuitem are OK

Could someone tell me what am i doing wrong? The osv.osv_memory object seems to be ok