fix prev

gui/qt/main_window.py

@@ -663,7 +663,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
             edit.setStyleSheet(ColorScheme.DEFAULT.as_stylesheet())
             fiat_e.is_last_edited = (edit == fiat_e)
             amount = edit.get_amount()
-            rate = self.fx.exchange_rate() if self.fx else None
+            rate = self.fx.exchange_rate() if self.fx else Decimal('NaN')
             if rate.is_nan() or amount is None:
                 if edit is fiat_e:
                     btc_e.setText("")