invoices: fix type hint for get_address()

electrum/invoices.py

@@ -117,7 +117,7 @@ class Invoice(StoredObject):
                 status_str = _('Expires') + ' ' + age(expiration, include_seconds=True)
         return status_str
 
-    def get_address(self) -> str:
+    def get_address(self) -> Optional[str]:
         """returns the first address, to be displayed in GUI"""
         if self.is_lightning():
             return self._lnaddr.get_fallback_address() or None

electrum/wallet.py

@@ -2474,7 +2474,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
         """Return the key to use for this invoice in self.receive_requests."""
         # FIXME: this should be a method of Invoice
         if not req.is_lightning():
-            addr = req.get_address()
+            addr = req.get_address() or ""
             if sanity_checks:
                 if not bitcoin.is_address(addr):
                     raise Exception(_('Invalid Bitcoin address.'))