qt: fix: addresses tab broken for imported watchonly wallets

fixes https://github.com/spesmilo/electrum/issues/10435

regression from https://github.com/spesmilo/electrum/pull/10376

electrum/gui/qt/address_list.py

@@ -25,7 +25,7 @@
 
 import enum
 from enum import IntEnum
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 
 from PyQt6.QtCore import Qt, QPersistentModelIndex, QModelIndex
 from PyQt6.QtGui import QStandardItemModel, QStandardItem, QFont
@@ -244,8 +244,14 @@ class AddressList(MyTreeView):
         # update counter
         self.num_addr_label.setText(_("{} addresses").format(num_shown))
 
-    def address_index_as_sortable_key(self, address_index: 'AddressIndexGeneric'):
-        return address_index if isinstance(address_index, str) else ''.join(f'{i:08x}' for i in address_index)
+    @staticmethod
+    def address_index_as_sortable_key(address_index: Optional['AddressIndexGeneric']) -> str:
+        if isinstance(address_index, str):  # pubkey hex
+            return address_index
+        elif address_index is None:
+            return ""
+        else:
+            return "".join(f"{i:08x}" for i in address_index)
 
     def refresh_row(self, key, row):
         assert row is not None

electrum/wallet.py

@@ -3784,7 +3784,10 @@ class Imported_Wallet(Simple_Wallet):
         return self.db.has_imported_address(address)
 
     def get_address_index(self, address) -> Optional[str]:
-        # returns None if address is not mine
+        # Return pubkey for address if we know it.
+        # If we don't know it, return None, which might happen:
+        # - if address is not is_mine
+        # - if this is an "imported address", we don't have the pubkey for. (watch-only imported wallet)
         return self.get_public_key(address)
 
     def get_address_path_str(self, address):