I already create a widget which contains button and onclick listener, but when I declare into tree view and upgrade the module the error always showup . 

File "/usr/lib/python3/dist-packages/odoo/addons/base/models/ir_ui_view.py", line 406, in _check_xml
    raise ValidationError(_('Invalid view %s definition in %s') % (view_name, view.arch_fs))
odoo.tools.convert.ParseError: "Invalid view crm.schedule.activity.tree.view (crm_integration.crm_schedule_activity_tree_view) definition in crm_integration/views/crm_schedule_activity_view.xml
None" while parsing /mnt/extra-addons/crm_integration/views/crm_schedule_activity_view.xml:2, near