RBF: make sure we know the fee for the old txn

related #4306

gui/qt/main_window.py

@@ -3138,6 +3138,9 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
 
     def bump_fee_dialog(self, tx):
         is_relevant, is_mine, v, fee = self.wallet.get_wallet_delta(tx)
+        if fee is None:
+            self.show_error(_("Can't bump fee: unknown fee for original transaction."))
+            return
         tx_label = self.wallet.get_label(tx.txid())
         tx_size = tx.estimated_size()
         d = WindowModalDialog(self, _('Bump Fee'))