files_Recursive fails when there are null values returned from files_And_Folders

[DinisCruz]

need to add a check to item to make sure it is value

    String::files_Recursive     = (extension)->
                                      files = []
                                      for item in @.str().files_And_Folders()
                                        if (item.is_Folder())
                                          files = files.concat(item.files_Recursive(extension))
                                        else
                                          if (not extension or item.file_Extension() is extension)
                                            files.push(item)
                                      return files