project_categ_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>

        <!-- Project form -->
        <record id="project_form_view_categs" model="ir.ui.view">
            <field name="name">project_form_view_categs</field>
            <field name="model">project.project</field>
            <field name="inherit_id" ref="project.edit_project"/>
            <field name="arch" type="xml">
                <field name="parent_id" position="after">
                        <field name="issue_categ_id" domain="[('child_ids','!=',False)]"/>
                </field>
            </field>
        </record>

        <!-- Project Issue form -->
        <record id="project_issue_form_view_categs" model="ir.ui.view">
            <field name="name">project_issue_form_view_categs</field>
            <field name="model">project.issue</field>
            <field name="inherit_id" ref="project_issue.project_issue_form_view"/>
            <field name="arch" type="xml">

                <field name="project_id" position="attributes">
                    <attribute name="on_change">onchange_project(project_id)</attribute>
                </field>

                <field name="project_id" position="after">
                    <field name="issue_categ_id" invisible="1"/>
                </field>

                <field name="categ_ids"  position="attributes">
                    <attribute name="attrs">{'required':[('issue_categ_id','!=',False)]}</attribute>
                </field>

            </field>
        </record>

    </data>
</openerp>