Working around bogus top-level folders in catalog

I have a catalog originally created on Mac and then moved to Windows, and it has two poison top-level folders that cause various SDK methods to fail.  For example, they cause catalog:setActiveSources({folder}) to have an assertion failure. 

For one of the folders, folder:getPath() returns nil. For another of the folders, folder:getPath() returns a Windows UNC file path that doesn't exist. I discovered this heuristic for identifying these bogus folders:

function isValidTopLevelFolder (folder)    local success, f = LrTasks.pcall (function ()        return catalog:getFolderByPath (folder:getPath ()) end)    return success and f    end

I've already provided this catalog to Becky Sowada of Adobe, following up on a bug report in the feedback forum. But I thought this work-around might be of usefulness to others someday.