exchange_rate: (fix) rm need to restart app to disable FX rates

Previously if the user disabled FX rates in the settings, the UI
would keep showing the fiat amounts everywhere until the next time
the program was started. (and the rates would not even refresh anymore)

electrum/exchange_rate.py

@@ -577,7 +577,7 @@ class FxThread(ThreadJob):
     def show_history(self):
         return self.is_enabled() and self.get_history_config() and self.ccy in self.exchange.history_ccys()
 
-    def set_currency(self, ccy):
+    def set_currency(self, ccy: str):
         self.ccy = ccy
         self.config.set_key('currency', ccy, True)
         self.trigger_update()
@@ -607,6 +607,8 @@ class FxThread(ThreadJob):
 
     def exchange_rate(self) -> Decimal:
         """Returns the exchange rate as a Decimal"""
+        if not self.is_enabled():
+            return Decimal('NaN')
         rate = self.exchange.quotes.get(self.ccy)
         if rate is None:
             return Decimal('NaN')