Odoo-Build/snippets/project.view_task_form2.parent_domain.xml

<?xml version="1.0"?>
<data inherit_id="project.view_task_form2" priority="50">

    <xpath expr="//field[@name='parent_id']" position="replace">
        <field name="parent_id" domain="[('parent_id', '=', False),('project_id', '=', project_id)]" attrs="{'invisible' : [('subtask_count', '&gt;', 0)]}" groups="project.group_subtask_project"/>
    </xpath>

</data>