Fix: Flash Builder 4 / VS2010: An internal error occured during:"Launching Main" java.lang.NullPointerException

After setting up Visual Studio 2010 and Flash Builder 4 on my new development machine, I found that I could not start my flex projects anymore. A click on the debug or run icon resulted in the following error message:

'Launching Main' has encountered a problem.An internal error occurred during "Launching Main".java.lang.NullPointerException

It turned out, that Flash Builder could not find the ASP.NET development server which I use to debug my flex-projects:

Flash Builder searches for C:\Windows\Microsoft.NET\Framework\v2.0.50727\WebDev.WebServer.exe, so I copied the files WebDev.WebServer20.exe and WebDev.WebServer20.exe.config from C:\Program Files (x86)\Common Files\microsoft shared\DevServer\10.0\ to C:\Windows\Microsoft.NET\Framework\v2.0.50727\ and renamed them to WebDev.WebServer.exe and WebDev.WebServer.exe.config.

Now everything runs fine again