After launching OpenBuildings Designer, a large grey window displays in front of the main application.

If I close this window then OpenBuildings Designer closes as well. I don't recall this happening before and cannot figure out how to correct it. 

Solution

You will notice that the main application window has a (1) in the title bar while the grey window in front has a (2).

This indicates that the "Open Multiple Application Windows" preference is enabled where (1) is the Primary application window and (2) the Secondary window.  By default that secondary grey window is typically on another monitor, but it can be dragged over top of the primary application window as is the case here. So you can simply drag that grey window to your second monitor, or if you do not need this feature you can disable it from user preferences.

See this wiki for more information about this option.