I sent a ticket in about this, but for those who are having the error message pop up when they try and open PG saying the application is running, try this:

-CLOSE YOUR BROWSER, THEN TRY TO OPEN PG

For some reason, when you use /wiki in the chat and it opens your browser, after you close PG, it appears linked to the wiki. So, close the browser and comment on this post. It works for me after I tried everything else, because it was just a waste of time to close Steam and re-start it all the time.

Thank you.