function getCMSFields() {
      $photoManager = new ImageDataObjectManager($this, "Photos", "FlameEventPage_Photo", "Photo", array("Title"=>"Title","Caption"=>"Caption"), "getCMSFields_forPopup");
      $photoManager->setUploadFolder(str_replace('assets/','',$this->AssetFolder()->Filename));
      $fields = parent::GetCMSFields();
      $fields->addFieldToTab("Root.Content.EventPhotos", $photoManager);
      return $fields;
   }

[Notice] Trying to get property of non-object
POST /SilverStripe/admin/EditForm/field/Photos/UploadForm?ctf[Photos][per_page]=10&ctf[Photos][showall]=0&ctf[Photos][sort]=Created&ctf[Photos][sort_dir]=DESC&ctf[Photos][search]=&ctf[Photos][filter]=&ctf[Photos][view]=grid&ctf[Photos][imagesize]=100

Line 317 in D:\wamp\www\silverstripe\dataobject_manager\code\FileDataObjectManager.php
Source

308    protected function getChildDataObj()
309    {
310       $class = $this->sourceClass();
311       return new $class();
312    }
313    
314    public function getPreviewFieldFor($fileObject, $size = 150)
315    {
316       if($fileObject instanceof Image) {
317          $URL = $fileObject->SetHeight($size)->URL;
318          return new LiteralField("icon",
319             "<div class='current-image'><img src='$URL' alt='' /><h3>$fileObject->Filename</h3></div>"
320          );
321       }
322       else {
323          $URL = $fileObject->Icon();         

Trace

* FileDataObjectManager->getPreviewFieldFor(Image)
Line 353 of FileDataObjectManager.php
* FileDataObjectManager->EditUploadedForm()
Line 303 of FileDataObjectManager.php
* FileDataObjectManager->saveUploadForm(Array,ImageDataObjectManager_Popup,HTTPRequest)
Line 228 of Form.php
* Form->httpSubmission(HTTPRequest)
Line 107 of RequestHandler.php
* RequestHandler->handleRequest(HTTPRequest)
Line 121 of RequestHandler.php
* RequestHandler->handleRequest(HTTPRequest)
Line 121 of RequestHandler.php
* RequestHandler->handleRequest(HTTPRequest)
Line 121 of RequestHandler.php
* RequestHandler->handleRequest(HTTPRequest)
Line 122 of Controller.php
* Controller->handleRequest(HTTPRequest)
Line 277 of Director.php
* Director::handleRequest(HTTPRequest,Session)
Line 121 of Director.php
* Director::direct(/admin/EditForm/field/Photos/UploadForm)
Line 115 of main.php