Today every time I attempted to debug my SharePoint workflow solution using Visual Studio 2008, I would receive the following error:
Failed to install the workflow template to Microsoft Office SharePoint Server.
Value cannot be null.
Parameter name: uriString
I could actually right click and run a deploy on the solution manually but I could not run in debug. It appears that the problem was caused by my install of WSPBuilder (http://wspbuilder.codeplex.com/) as when I disabled it from startup and resaved my SharePoint workflow project in Visual Studio, the debugger started working. A simple restart of VS 2008 did not fix the issue.