I have a piece of Python code that redirects the user to another view when he/she clicks a button. The redirecting is done as follows:

    view_ref = self.pool.get('ir.model.data').get_object_reference(cr, uid, 'hr_timesheet', 'hr_timesheet_line_tree')
    view_id = view_ref and view_ref[1] or False

    return {
        'type': 'ir.actions.act_window',
        'name': 'Timesheets',
        'view_mode': 'tree',
        'view_id': view_id,
        'res_model': 'hr.analytic.timesheet',
        'context': context,
     }

This opens the new view into the current window like this:

That works fine, but it would be better if the window opened in an overlay instead. I then added 'target':'new' to the return statement's dictionary, and the result appears like this:

So the overlay opens up, but the create button is missing so you can only access the current rows, not add new ones. Any ideas what could be causing this?