project_issue_view.xml

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

        <!--PROJECT ISSUE FORM -->
        <record id="project_issue_form_view_task" model="ir.ui.view">
            <field name="name">project_issue_form_view_task</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="task_id" position="after">
                    <button type="object" name="action_create_task" string="Task Report" attrs="{'invisible': [('task_id', '!=', False)]}"/>
               </field>

            </field>
        </record>

    </data>
</openerp>