More thread safety.

FileManager/BackgroundFileSystem.cs

@@ -94,20 +94,26 @@ namespace FileManager
         }
 
         private void RemovePath(string path)
+        {
+            lock (fsCache)
             {
                 var pathsToRemove = fsCache.Where(p => p.StartsWith(path)).ToArray();
 
                 foreach (var p in pathsToRemove)
                     fsCache.Remove(p);
             }
+        }
 
         private void AddPath(string path)
+        {
+            lock (fsCache)
             {
                 if (File.GetAttributes(path).HasFlag(FileAttributes.Directory))
                     fsCache.AddRange(Directory.EnumerateFiles(path, SearchPattern, SearchOption));
                 else
                     fsCache.Add(path);
             }
+        }
 
         #endregion
     }