"There is an error" when clicking on some Silverstripe admin pages - php

Question

I'm trying to fix a problem with some Silverstripe admin pages. Everytime when I click on Cart or Example product page, the message - "There is an error" pops up, and the page wouldn't show. Please see the attached image.
As you can see page Cart and example product's icons are different from the rest. I didn't write the code myself and I've never experienced this before, so any suggestion on where I should start to tackle the problem would be appreciated.
I can copy some code here if you can tell me which part. Thank you very much for your time.
Regards
Sam
Firefox console message when click on the Cart page.
Additional error message under console response tab:
ERROR [User Error]: Bad class to singleton() - ProductImageObject
IN POST /admin/getitem?ID=17&ajax=1
Line 334 in /home/xxx/subdomains/xxx/sapphire/core/Core.php
Source
======
325: *
326: * #param string $className
327: * #return Object
328: */
329: function singleton($className) {
330: global $_SINGLETONS;
331: if(!isset($className)) user_error("singleton() Called without a class", E_USER_ERROR);
332: if(!is_string($className)) user_error("singleton() passed bad class_name: " .
var_export($className,true), E_USER_ERROR);
333: if(!isset($_SINGLETONS[$className])) {
* 334: if(!class_exists($className)) user_error("Bad class to singleton() - $className",
E_USER_ERROR);
335: $_SINGLETONS[$className] = Object::strong_create($className,null, true);
336: if(!$_SINGLETONS[$className]) user_error("singleton() Unknown class '$className'", E_USER_ERROR);
337: }
338: return $_SINGLETONS[$className];
339: }
340:
Trace
=====
<ul>user_error(Bad class to singleton() - ProductImageObject,256)
line 334 of Core.php
singleton(ProductImageObject)
line 96 of DataObjectManager.php
DataObjectManager->__construct(Product,ProductImages,ProductImageObject,Array,getCMSFields_forPopup,,Created DESC,)
line 48 of FileDataObjectManager.php
FileDataObjectManager->__construct(Product,ProductImages,ProductImageObject,ProductImage,Array,getCMSFields_forPopup)
line 125 of Product.php
Product->getCMSFields(CMSMain)
line 444 of CMSMain.php
CMSMain->getEditForm(17)
line 1021 of LeftAndMain.php
LeftAndMain->EditForm()
line 382 of LeftAndMain.php
LeftAndMain->getitem(SS_HTTPRequest)
line 193 of Controller.php
Controller->handleAction(SS_HTTPRequest)
line 137 of RequestHandler.php
RequestHandler->handleRequest(SS_HTTPRequest)
line 147 of Controller.php
Controller->handleRequest(SS_HTTPRequest)
line 281 of Director.php
Director::handleRequest(SS_HTTPRequest,Session)
line 124 of Director.php
Director::direct(/admin/getitem)
line 127 of main.php
</ul>

Answer 1

This can have many reasons, try 2 things to get started:
1) Put your site into development mode (if it isn't already): http://doc.silverstripe.org/sapphire/en/topics/debugging#dev-mode
2) Log errors (server-side): http://doc.silverstripe.org/sapphire/en/topics/error-handling#filesystem-logs
This will help you with server-side errors (and "there has been an error/500" sounds a lot like it) - you should find an entry related to it in the log file. If it's just a client-side / JS thing, you'll need to dive deeper into firebug...
And don't worry about the icons in the page tree, this is a feature (see http://www.ssbits.com/snippets/2009/spice-up-your-cms-sitetree/ for a brief description).

Answer 2

From what I see, you have to check the PHP class that defines your Cart Page. Most probably, you have misspelled or mistyped something.
Bad class to singleton() - ProductImageObject
This is our clue. Try checking line 125 of Product.php.