Error: File Manager is currently unavailable. DotNetNuke.Services.Exceptions.ModuleLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Services.FileSystem.FolderProvider.Instance(String friendlyName) at DotNetNuke.Services.FileSystem.FolderMappingInfo.get_ImageUrl() at DotNetNuke.Modules.Admin.FileManager.FileManager.PreloadFolderImages() at DotNetNuke.Modules.Admin.FileManager.FileManager.InitializeTree() at DotNetNuke.Modules.Admin.FileManager.FileManager.OnLoad(EventArgs e) --- End of inner exception stack trace ---

SOLUTION:

1. Open the file in a text editor DesktopModules/Admin/FileManager/FileManager.aspx.cs 
2. Locate the function : PreloadFolderImages()
3. and make the following highlighted changes.
   

private void PreloadFolderImages()
        {
            IFolderMappingController folderMappingController = FolderMappingController.Instance;
            FolderMappingInfo folderMappingInfo = null;
            string imageUrl = String.Empty;

            foreach (var folderMapping in FolderMappings)
            {
                try
                {
                folderMappingInfo = folderMappingController.GetFolderMapping(folderMapping.Key);
                imageUrl = folderMappingInfo.ImageUrl;
                DNNTree.ImageList.Add(imageUrl);
                }
                catch {}
            }
        }