Today morning I am just open Visual Studio 2010 and create a new Windows phone project and came across an issue with windows phone xaml page with the following error in design view.
Value cannot be null.Parameter
name: parentContext in Windows Phone 7
To rectify this issue I did two worked around and both are worked.
1) Build your project and close the page windows and then open.
2) Close the Visual Studio IDE and re-launch it.
Hope it helps.