Bug fix: setVisible(..) doesn't take None values (#969)
Bug fix: setVisible(..) doesn't take None values (#969)

This led to the following error with a fresh wallet when clicking the
"Clear" button on the send tab:

Traceback (most recent call last):
  File "/home/awemany/Electron-Cash/gui/qt/util.py", line 590, in on_done
    cb(result)
  File "/home/awemany/Electron-Cash/gui/qt/main_window.py", line 1620, in broadcast_done
    self.do_clear()
  File "/home/awemany/Electron-Cash/gui/qt/main_window.py", line 1739, in do_clear
    self.message_opreturn_e.setVisible(self.config.get('enable_opreturn'))
TypeError: setVisible(self, bool): argument 1 has unexpected type 'NoneType'
fyookball/electrum,fyookball/electrum,fyookball/electrum