Here’s a quick tip. If you’ve used nested master pages in your ASP.Net web application and you’ve tried to enter the “Design view” in one of your nested aspx pages, you’ve probably seen this error message.
Design view does not support creating or editing nested master pages. To create or edit nested master pages, use Source view.
A simple trick to side step this issue is to clear the MasterPageFile attribute in your aspx file’s page declaration.
So this: MasterPageFile=”~/Example/Example.Master”
Becomes this: MasterPageFile=””
Now you can edit your page using the Design view. Just remember to put the MasterPageFile reference back when you’re ready to test or deploy your site.