There is a problem with the popup window, I get it too, but the actual GenXML process should complete as expected. If you look in your history window you should see a green gear icon labelled "WMC.GenXML" and if you roll over it you should see the report that was supposed to appear in the popup window telling you what was found.
Look in the .\wmcPayloads folder and you should see your 3 xml files. These will be automatically used in your commands if you selected the WMC preset configuration for VC.
I don't think there are many people still using WMC, especially not VC users, or surely someone would have reported the blank window issue before now.
I have also noticed that the GenXML for WMC only seems to work the first time you run it, after that it will return 0 items. I will look into it, but for now you only need to restart VC and then the GenXML will work again. Anyway, you only need to run it once in a while when you have added new music to your library.