Problem

An Unhandled Exception has occurred when you start of developing Silverlight Application

Meaning Of Error

If you are new to silverlight and When you start of developing silverlight application in VS 2010. if you ever see the xaml page like below. be cool and follow the steps mentioned below.

Solution

The reason is Silverlight Development runtime and development sdk are in different versions or you have installed more than one silverlight versions in your machine. if you see below screen shot last column shows the version (in "program and features" in your control panel). There sdk version and silverlight runtime are in different versions.
So keep both are in same version and then you are not getting above error anymore.

Conclusion

Hope this will resolve your problem.