fix error on python 2.7.6 : TypeError: coercing to Unicode

The log is：
Traceback (most recent call last):
  File "/home/wbl/dev/app/network/huhamhire-hosts/gui/qdialog_slots.py", line 228, in on_MakeHosts_clicked
    self.make_hosts("system")
  File "/home/wbl/dev/app/network/huhamhire-hosts/gui/qdialog_d.py", line 204, in make_hosts
    self.set_config_bytes(mode)
  File "/home/wbl/dev/app/network/huhamhire-hosts/gui/qdialog_d.py", line 282, in set_config_bytes
    if (self.custom == None and os.path.isfile(self.custom)):
  File "/usr/lib/python2.7/genericpath.py", line 29, in isfile
    st = os.stat(path)
TypeError: coercing to Unicode: need string or buffer, NoneType found