Notice: Trying to get property of non-object in C:\Projects\myproject\src\sapphire\forms\FileIFrameField.php on line 18

<?php

class Project extends Page
{
   static $db = array(
   );

   static $has_one = array(
   );
   static $has_many = array(
      'ProjectAssets' => 'ProjectAsset',
   );

   function getCMSFields()
   {
      $fields = parent::getCMSFields();

      $tablefield = new HasManyComplexTableField(
            $this,
            'ProjectAssets',
          'ProjectAsset',
          array(
             'FileTitle' => 'Title'
          ),
          'getCMSFields_forPopup'
      );
      $tablefield->setAddTitle('file');
      $fields->addFieldToTab( 'Root.Content.Files', $tablefield );

      return $fields;
   }
}

class Project_Controller extends Page_Controller
{
}

?>
<?php

class ProjectAsset extends DataObject {
   static $db = array(
      'FileTitle' => 'Varchar(200)'
   );
static $has_one = array(
      'Project' => 'Project',
      'File' => 'File'      
   );
   function getCMSFields_forPopup() {
      return new FieldSet(
         new TextField('FileTitle', 'Title'),
         new FileIFrameField('File')
      );
   }
}

?>