The bug was reported in the ns-3-users mailing list here 
https://groups.google.com/d/msg/ns-3-users/3mxFpajyu_o/ezZfSQ2cBAAJ

The python script will try to calculate the rxBitrate as follows:
        if rx_duration > 0:
            self.rxBitrate = long(flow_el.get('rxBytes'))*8 / rx_duration
        else:
            self.rxBitrate = None

If you do not receive any packet, it will be assigned 'None' which does not print as float and hence the error.
      
print "\tRX bitrate: %.2f kbit/s" % (flow.rxBitrate*1e-3,)

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The fix should be trivial to have a check before the print.