Getting Error while using web browser with HTML Element Set Attribute in Framework 4.0 in C#

hello,

I am using Web browser control to replace Rich Text Editor for getting HTML output. Everything works fine in framework 2.0

but when i am trying to change  to framework 4.0 then gives error 

At 

 webBrowser1.Document.Write(webBrowser1.DocumentText);
            doc.designMode = "On";
            webBrowser1.Document.Body.SetAttribute("contentEditable", "true");

Error is "Object reference not set to an instance of an object."

Reply ASAP,

Regards,

Divyang