ERROR:RELAXNGV:RELAXNG_ERR_EXTRACONTENT: Element form has extra content: sheet

I'm going to developer work offer module.in that module if we selected the relevant division we can select workers for work offer for them (in plantation industry). tried with comparing lunch module (lunch order form) but i got this error. (OpenERP ver 7)

here is my view.xml

    <record model="ir.ui.view" id="bpl_work_offer_form">
        <field name="name">bpl.work.offer.form</field>
        <field name="model">bpl.work.offer</field>
        <field name="arch" type="xml">
            <form string='bpl_work_offer'>
                <sheet>
                    <group>
                        <group>
                            <field name='user_id' />
                        </group>
                        <group>
                            <field name='date_of_offer' />
                        </group>
                    </group>
                    <div name="Worker Selection"></div>
                    <separator string='Select workers' />
                    <field name='selected_workers_line_ids' nolabel='1'>
                        <tree string='List' editable='bottom'>
                            <field name='worker_id' />
                            <field name='is_selected' />
                        </tree>
                    </field>
                    <group class='oe_subtotal_footer oe_right'>
                        <field name='total_workers' />
                    </group>
                    <br />
                    <br />
                </sheet>
            </form>
        </field>
    </record>

here is my model class

class bpl_work_offer(osv.osv):
    _name = "bpl.work.offer"
    _description = "BPL Work Offer"
    _columns = {
        'user_id': fields.many2one('res.users', 'User Name'),
        'date_of_offer': fields.date('Date'),
        'bpl_company_id':fields.many2one('bpl.company.registration', 'Company', help='Company'),
        'bpl_estate_id':fields.many2one('bpl.estate.registration', 'Estate', help='Estate'),
        'bpl_division_id':fields.many2one('bpl.division.registration', 'Division', help='Division'),
        'payment_type': fields.selection([('normal_work', 'Normal Work'), ('cash_work', 'Cash Work'), ('contract_work', 'Contract Work')], 'Payment Type'),
        'select_by': fields.selection([('by_name', 'By Names'), ('by_count', 'By Count')], 'Select By'),
        'work_type': fields.selection([('tea', 'Tea'), ('rubber', 'Rubber'), ('sundry', 'Sundry')], 'Work Type'),
        'total_workers': fields.integer('Total Workers'),
        'selected_workers_line_ids': fields.one2many('selected.workers.line', 'worker_id', 'Workers', ondelete="cascade"),
    }

bpl_work_offer()

class selected_workers_line_ids(osv.osv):
    _name = 'selected.workers.line'
    _description = 'lunch order line'
    _columns = {
        'worker_id':fields.many2one('bpl.worker', 'Worker', help='Worker'),
        'is_selected': fields.boolean('Select', help="Selected or not"),
        }

selected_workers_line_ids()

my error is like this

2013-03-21 06:31:50,137 7539 ERROR bpl openerp.addons.base.ir.ir_ui_view: <string>:2:0:ERROR:RELAXNGV:RELAXNG_ERR_EXTRACONTENT: Element form has extra content: sheet
2013-03-21 06:31:50,164 7539 ERROR bpl openerp.tools.convert: Parse error in /home/priyan/Software/openerp-7.0-20130309-002120/openerp/addons/bpl/bpl_view.xml:276:
<record model="ir.ui.view" id="bpl_work_offer_form">
            <field name="name">bpl.work.offer.form</field>
            <field name="model">bpl.work.offer</field>
            <field name="arch" type="xml">
                <form string="bpl_work_offer">
........
  File "/home/priyan/Software/openerp-7.0-20130309-002120/openerp/osv/orm.py", line 1544, in _validate
    raise except_orm('ValidateError', '\n'.join(error_msgs))
except_orm: ('ValidateError', u'Error occurred while validating the field(s) arch: Invalid XML for View Architecture!')
2013-03-21 06:31:50,185 7539 INFO bpl werkzeug: 127.0.0.1 - - [21/Mar/2013 06:31:50] "POST /web/session/authenticate HTTP/1.1" 200 -