Here I have a question about odoo upgrade

File "E:\ailsaODOO\odoo\odoo\addons\base\ir\ir_ui_view.py", line 342, in _check_xml

  raise ValidationError(_('Invalid view definition'))

odoo.tools.convert.ParseError: "Invalid view definition

None" while parsing file:/e:/ailsaodoo/odoo/addons/ailsaodoo/views/ailsaodoo.xml:88, near

<record model="ir.ui.view" id="session_tree_view">

  <field name="name">session.tree</field>

  <field name="model">ailsaodoo.session</field>

  <field name="arch" type="xml">

  <tree strin="session tree">

  <field name="name"/>

  <field name="course_id"/>

  </tree>

  </field>

  </record>

every thing is ok before I want to upgrade my module, and I cannot figure it out the problem