Unable to attach to application ‘WebDev.WebServer.EXE’

I was working on an ASP.NET project today, and suddenly this error came up after I click “Debug”:

Unable to attach to ‘WebDev.WebServer.EXE’ (PID: xxxx) using ‘ComputerName’. A debugger is already attached. Do you want to continue anyway?

If I click Yes, my browser becomes loading forever.

The cause I believe is that the built-in ASP.NET development server in Visual Studio is crashed, but somehow the process is still attached to the debugger. I tried to close the development server by right-clicking the icon in the taskbar area, but no menu popped up for me to close it, so, I had to close the process in Task Manager.

After I killed the process, the error went away.