Every once in a while, SourceGen will become unresponsive when it
tries to show a MessageBox. In the debugger you can see the GC
running frantically, but the stack trace is just sitting on the
MessageBox show call. Apparently, if you don't specify a parent
window argument, the MessageBox will occasionally end up behind
everything else, and you can get stuck.
I'm not sure what the GC frenzy is about, or whether this will fix
what I'm seeing, but it's easy to do and might solve the problem.
cf. https://stackoverflow.com/q/3467403/294248
