fix: network.interface can become None at any time

gui/kivy/main_window.py

@@ -639,8 +639,9 @@ class ElectrumWindow(App):
         chain = self.network.blockchain()
         self.blockchain_checkpoint = chain.get_checkpoint()
         self.blockchain_name = chain.get_name()
-        if self.network.interface:
-            self.server_host = self.network.interface.host
+        interface = self.network.interface
+        if interface:
+            self.server_host = interface.host
 
     def on_network_event(self, event, *args):
         Logger.info('network event: '+ event)

gui/qt/network_dialog.py

@@ -340,7 +340,8 @@ class NetworkChoiceLayout(object):
         self.server_port.setText(port)
         self.autoconnect_cb.setChecked(auto_connect)
 
-        host = self.network.interface.host if self.network.interface else _('None')
+        interface = self.network.interface
+        host = interface.host if interface else _('None')
         self.server_label.setText(host)
 
         self.set_protocol(protocol)

lib/synchronizer.py

@@ -126,7 +126,9 @@ class Synchronizer(ThreadJob):
         tx_fees = dict(filter(lambda x:x[1] is not None, tx_fees))
         # Note if the server hasn't been patched to sort the items properly
         if hist != sorted(hist, key=lambda x:x[1]):
-            self.network.interface.print_error("serving improperly sorted address histories")
+            interface = self.network.interface
+            # note: we don't actually know which interface was used if it was *just* changed
+            if interface: interface.print_error("serving improperly sorted address histories")
         # Check that txids are unique
         if len(hashes) != len(result):
             self.print_error("error: server history has non-unique txids: %s"% addr)