error when flush output to ProgressLogger

The ProgressLogger "out_filename" is set to null by default . When flush the output by "self.out_file.flush()" in the ProgressLogger() function, an error occurs: AttributeError: 'NoneType' object has no attribute 'flush'

Simple workaround could be either giving a filename to ProgressLogger "out_filename" or disabling "self.out_file.flush()".