qml: add QEAmount.copyFrom(QEAmount) so we can take a new amount object without changing the instance.

This avoids crashes when a QEAmount is already referenced in QML, and then replaced with another instance (e.g. in qetxfinalizer.py)

electrum/gui/qml/qetxfinalizer.py

@@ -74,7 +74,7 @@ class QETxFinalizer(QObject):
     def amount(self, amount):
         if self._amount != amount:
             self._logger.debug(str(amount))
-            self._amount = amount
+            self._amount.copyFrom(amount)
             self.amountChanged.emit()
 
     effectiveAmountChanged = pyqtSignal()
@@ -90,7 +90,7 @@ class QETxFinalizer(QObject):
     @fee.setter
     def fee(self, fee):
         if self._fee != fee:
-            self._fee = fee
+            self._fee.copyFrom(fee)
             self.feeChanged.emit()
 
     feeRateChanged = pyqtSignal()

electrum/gui/qml/qetypes.py

@@ -77,6 +77,11 @@ class QEAmount(QObject):
     def isEmpty(self):
         return not(self._is_max or self._amount_sat or self._amount_msat)
 
+    def copyFrom(self, amount):
+        self.satsInt = amount.satsInt
+        self.msatsInt = amount.msatsInt
+        self.isMax = amount.isMax
+
     def __eq__(self, other):
         if isinstance(other, QEAmount):
             return self._amount_sat == other._amount_sat and self._amount_msat == other._amount_msat and self._is_max == other._is_max