fee ui qt: fee_e and feerate_e was sometimes not getting filled

gui/qt/main_window.py

@@ -1329,8 +1329,13 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
             # actual fees often differ somewhat.
             if freeze_feerate or self.fee_slider.is_active():
                 displayed_feerate = self.feerate_e.get_amount()
-                displayed_feerate = displayed_feerate // 1000 if displayed_feerate else 0
-                displayed_fee = displayed_feerate * size
+                if displayed_feerate:
+                    displayed_feerate = displayed_feerate // 1000
+                else:
+                    # fallback to actual fee
+                    displayed_feerate = fee // size if fee is not None else None
+                    self.feerate_e.setAmount(displayed_feerate)
+                displayed_fee = displayed_feerate * size if displayed_feerate is not None else None
                 self.fee_e.setAmount(displayed_fee)
             else:
                 if freeze_fee: