the detail message is Value cannot be null when impletement ArcGIS silverlight API

[LuChen]

we are doing a silverlight project and ArcGIS is part of it. when I tried to intergrid my silverlight project into our whole project ,which is build on Devpress and Caliburn.Micro. I got the error mesasge :

anyone has clue about how to solve the problem?

[LuChen]

Problem fixed:

Solution. I was setting the extent at the runtime, which caused the above problem. So I set the extent in XAML file to let it run in the design time. the problem is gone.