Error when a page is edited

Hi,
I am migrating a website from SF 3.7 to 5. 
Everything was working fine. But now the site is showing error when I try to edit the page in admin site. Also, this error is not there for all pages. I couldn't track out by debugging.
Value cannot be null.
Parameter name: key.
The error seems to occur for some pages that are not shown in menu.
When the page is open in edit mode, it shows null error in menu controls section. 
Is it related to radcontrol because both these are radcontrols. 
What is causing this error. Please help. 

Thanks.