Value Cannot be be null. Parameter name: name on VS SSRS Solution

What could be problem?

When I open VS2008, I get Error box "Value Cannot be be null. Parameter name: name" and I see a Report Design in backgroud. (this reports have no parameter at all)

We have been developing SQL Server 2008R2 SSRS reports.

I have several reports in this VS Solution. Some of the reports do have parameters and they are cascading parameters.

All reports looks good despite Value cannot be null error.

Kenny_I