kivy swaps: handle no dynamic fee estimates

fixes #7215

electrum/gui/kivy/uix/dialogs/lightning_channels.py

@@ -680,12 +680,12 @@ class SwapDialog(Factory.Popup):
 
     def update_fee_text(self):
         fee_per_kb = self.config.fee_per_kb()
+        fee_per_b = format_fee_satoshis(fee_per_kb / 1000) if fee_per_kb is not None else "unknown"
         # eta is -1 when block inclusion cannot be estimated for low fees
         eta = self.config.fee_to_eta(fee_per_kb)
 
-        fee_per_b = format_fee_satoshis(fee_per_kb / 1000)
         suggest_fee = self.config.eta_target_to_fee(RECOMMEND_BLOCKS_SWAP)
-        suggest_fee_per_b = format_fee_satoshis(suggest_fee / 1000)
+        suggest_fee_per_b = format_fee_satoshis(suggest_fee / 1000) if suggest_fee is not None else "unknown"
 
         s = 's' if eta > 1 else ''
         if eta > RECOMMEND_BLOCKS_SWAP or eta == -1:

electrum/simple_config.py

@@ -405,12 +405,17 @@ class SimpleConfig(Logger):
             return 1
         return FEE_ETA_TARGETS[slider_pos]
 
-    def fee_to_eta(self, fee_per_kb: int) -> int:
+    def fee_to_eta(self, fee_per_kb: Optional[int]) -> int:
         """Returns 'num blocks' ETA estimate for given fee rate,
         or -1 for low fee.
         """
         import operator
-        lst = list(self.fee_estimates.items()) + [(1, self.eta_to_fee(len(FEE_ETA_TARGETS)))]
+        lst = list(self.fee_estimates.items())
+        next_block_fee = self.eta_target_to_fee(1)
+        if next_block_fee is not None:
+            lst += [(1, next_block_fee)]
+        if not lst or fee_per_kb is None:
+            return -1
         dist = map(lambda x: (x[0], abs(x[1] - fee_per_kb)), lst)
         min_target, min_value = min(dist, key=operator.itemgetter(1))
         if fee_per_kb < self.fee_estimates.get(FEE_ETA_TARGETS[0])/2: