CORE-4606 SelectFolderByIdLoadHandler fixes.

SelectFolderByIdLoadHandler#onLoad now sets the rootsLoaded flag before calling initPathsToLoad, since that method may attempt to load sub-folders, which may not be an async call now, so the onLoad method may be called again before initPathsToLoad returns.
Prevent potential NPE in initPathsToLoad.(cherry picked from commit 5a5c2bc318038caacf980335fc7299409f9d647d)