<?php
class Calltoaction extends DataObject
{
   static $has_one = array (
      'Attachment' => 'File',
      'InternalLink' => 'SiteTree',
      'StaticSidebar' => 'StaticSidebar'
   );

   public function getCMSFields_forPopup()
   {
      return new FieldSet(
         new TreeDropdownField("InternalLinkID", "Choose a page which this call to action links to:", "SiteTree"),
         new FileIFrameField('Attachment')
      );
   }
}

<?php
class StaticSidebar extends Page
{
   static $has_many = array (
      'Calltoactions' => 'Calltoaction'
   );
   public function getCMSFields()
   {
      $fields = parent::getCMSFields();
      $manager = new ImageDataObjectManager(
         $this, // Controller
         'Calltoactions', // Source name
         'Calltoaction', // Source class
         'Attachment', // File name on DataObject
         array(
            'InternalLink' => 'InternalLink'
         ), // Headings
         'getCMSFields_forPopup' // Detail fields (function name or FieldSet object)
         // Filter clause
         // Sort clause
         // Join clause
      );
      $fields->addFieldToTab("Root.Content.Sidebar Images",$manager);
      return $fields;
   }
   static $allowed_children = array("none");
}

class StaticSidebar_Controller extends Page_Controller {

public static $allowed_actions = array (
);

public function init() {
parent::init();
}
}

http://19thholegolfgetaways.co.uk/admin/EditForm/field/Calltoactions?SecurityID=e9ea1a8fafe6d260fcc0fe550181506c69f50cf7&ctf[Calltoactions][start]=0&ctf[Calltoactions][per_page]=10&ctf[Calltoactions][showall]=0&ctf[Calltoactions][sort]=&ctf[Calltoactions][sort_dir]=&ctf[Calltoactions][search]=&ctf[Calltoactions][filter]=&ctf[Calltoactions][view]=grid&ctf[Calltoactions][imagesize]=100

ERROR [User Error]: Uncaught Exception: Object->__call(): the method 'fortemplate' does not exist on 'SiteTree'
IN GET /admin/EditForm/field/Calltoactions?SecurityID=e9ea1a8fafe6d260fcc0fe550181506c69f50cf7&ctf[Calltoactions][start]=0&ctf[Calltoactions][per_page]=10&ctf[Calltoactions][showall]=0&ctf[Calltoactions][sort]=&ctf[Calltoactions][sort_dir]=&ctf[Calltoactions][search]=&ctf[Calltoactions][filter]=&ctf[Calltoactions][view]=grid&ctf[Calltoactions][imagesize]=100
Line 724 in /var/sites/1/19thholegolfgetaways.co.uk/public_html/sapphire/core/Object.php

Source
======
715:             
716:             default :
717:                throw new Exception (
718:                   "Object->__call(): extra method $method is invalid on $this->class:" . var_export($config,
true)
719:                );
720:          }
721:       } else {
722:          // Please do not change the exception code number below.
723:          
* 724:          throw new Exception("Object->__call(): the method '$method' does not exist on '$this->class'",
2175);
725:       }
726:    }
727:    
728:    //
-----------------------------------------------------------------------------------------------------------------
729:    
730:    /**

Trace
=====
<ul>Object->__call(forTemplate,Array)

SiteTree->forTemplate()
line 447 of ViewableData.php

ViewableData->XML_val(InternalLink)
line 1367 of TableListField.php

TableListField_Item->Fields()
line 651 of DataObjectManager.php

DataObjectManager_Item->Fields()
line 369 of ViewableData.php

ViewableData->obj(Fields)
line 434 of .cache.dataobject_manager.templates.ImageDataObjectManager.ss

include(/var/sites/1/19thholegolfgetaways.co.uk/public_html/silverstripe-cache/.cache.dataobject_manager.templates.ImageDataObjectManager.ss)
line 420 of SSViewer.php

SSViewer->process(ImageDataObjectManager)
line 342 of ViewableData.php

ViewableData->renderWith(ImageDataObjectManager)
line 275 of ComplexTableField.php

ComplexTableField->FieldHolder()
line 282 of DataObjectManager.php

DataObjectManager->FieldHolder()
line 281 of TableListField.php

TableListField->index(SS_HTTPRequest)
line 143 of RequestHandler.php

RequestHandler->handleRequest(SS_HTTPRequest)
line 161 of RequestHandler.php

RequestHandler->handleRequest(SS_HTTPRequest)
line 161 of RequestHandler.php

RequestHandler->handleRequest(SS_HTTPRequest)
line 147 of Controller.php

Controller->handleRequest(SS_HTTPRequest)
line 282 of Director.php

Director::handleRequest(SS_HTTPRequest,Session)
line 125 of Director.php

Director::direct(/admin/EditForm/field/Calltoactions)
line 127 of main.php