qt main_window: do not unregister on shutdown

this is handled differently in lightning, see commit

commit 6e355601261a60d143561f15760cc48f9c81d000
Author: ThomasV <thomasv@electrum.org>
Date:   Sun Jun 3 10:07:56 2018 +0200

    integrate channels_list with existing framework

electrum/gui/qt/main_window.py

@@ -3354,8 +3354,6 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         self.wallet.thread.stop()
         if self.network:
             self.network.unregister_callback(self.on_network)
-            self.network.unregister_callback(self.on_quotes)
-            self.network.unregister_callback(self.on_history)
         self.config.set_key("is_maximized", self.isMaximized())
         if not self.isMaximized():
             g = self.geometry()

electrum/lnrouter.py

@@ -354,7 +354,7 @@ class ChannelDB(JsonDB):
 
     def capacity(self):
         # capacity of the network
-        return sum(c.capacity_sat for c in self._id_to_channel_info.values())
+        return sum(c.capacity_sat for c in self._id_to_channel_info.values() if c.capacity_sat is not None)
 
     def get_channel_info(self, channel_id: bytes) -> Optional[ChannelInfo]:
         return self._id_to_channel_info.get(channel_id, None)