PHP error when entering each minipanel "content edit" page when the minipanel is used in site_template

When entering the "content edit" page of a minipanel which is used in site_template i got a PHP error:
Fatal error: Call to undefined method stdClass::render() in panels.module on line 1013

these are lines 1000-1014 of panels.module

function panels_render_display(&$display, $renderer = NULL) {   
  ctools_include('display-render', 'panels');    
  ctools_include('plugins', 'panels');   
  ctools_include('context');   
    
  if (!empty($display->context)) {   
    if ($form_context = ctools_context_get_form($display->context)) {  
      $form_context->form['#theme'] = 'panels_render_display_form';   
      $form_context->form['#display'] = &$display;   
      $form_context->form['#form_context_id'] = $form_context->id;   
      return drupal_render_form($form_context->form_id, $form_context->form);   
    }   
  }   
  return $display->render($renderer);   
}