Fix #1788 - Defensive paranoia against self.recievie_address == None
Fix #1788 - Defensive paranoia against self.recievie_address == None

This shouldn't normally happen now that we disllow user deleting last
address in the wallet, but the fact that the main_window class can be
initialized with self.receive_address = None, and later we never check
if it's None, is sloppy.  This commit addresses that and closes #1788.

fyookball/electrum,fyookball/electrum,fyookball/electrum