wallet: (fix) get_receiving_address must always return an addr

- also, disallow deleting last address from an imported wallet (fixes #3254, fixes #4833)
- also, set LNBackups.sweep_address lazily, as during fresh wallet creation
  there are no addresses in the wallet at that point yet! see trace below.

Traceback (most recent call last):
  [...]
  File "...\electrum\electrum\tests\test_commands.py", line 112, in test_export_private_key_deterministic
    wallet = restore_wallet_from_text('bitter grass shiver impose acquire brush forget axis eager alone wine silver',
  File "...\electrum\electrum\wallet.py", line 2575, in restore_wallet_from_text
    wallet = Wallet(db, storage, config=config)
  File "...\electrum\electrum\wallet.py", line 2502, in __new__
    wallet = WalletClass(db, storage, config=config)
  File "...\electrum\electrum\wallet.py", line 2346, in __init__
    Deterministic_Wallet.__init__(self, db, storage, config=config)
  File "...\electrum\electrum\wallet.py", line 2147, in __init__
    Abstract_Wallet.__init__(self, db, storage, config=config)
  File "...\electrum\electrum\wallet.py", line 261, in __init__
    self.lnbackups = LNBackups(self)
  File "...\electrum\electrum\lnworker.py", line 1401, in __init__
    self.sweep_address = wallet.get_receiving_address()
  File "...\electrum\electrum\wallet.py", line 1498, in wrapper
    addr = func(self, *args, **kwargs)
  File "...\electrum\electrum\wallet.py", line 1520, in get_receiving_address
    raise Exception("no receiving addresses in wallet?!")
Exception: no receiving addresses in wallet?!

electrum/lnworker.py

@@ -1398,11 +1398,14 @@ class LNBackups(Logger):
         self.lock = threading.RLock()
         self.wallet = wallet
         self.db = wallet.db
-        self.sweep_address = wallet.get_receiving_address()
         self.channel_backups = {}
         for channel_id, cb in self.db.get_dict("channel_backups").items():
             self.channel_backups[bfh(channel_id)] = ChannelBackup(cb, sweep_address=self.sweep_address, lnworker=self)
 
+    @property
+    def sweep_address(self) -> str:
+        return self.wallet.get_receiving_address()
+
     def channel_state_changed(self, chan):
         util.trigger_callback('channel', chan)
 

electrum/wallet.py

@@ -1513,11 +1513,11 @@ class Abstract_Wallet(AddressSynchronizer, ABC):
             return addrs[0]
 
     @check_returned_address
-    def get_receiving_address(self):
+    def get_receiving_address(self) -> str:
         # always return an address
         domain = self.get_receiving_addresses()
         if not domain:
-            return
+            raise Exception("no receiving addresses in wallet?!")
         choice = domain[0]
         for addr in domain:
             if not self.is_used(addr):
@@ -2030,6 +2030,8 @@ class Imported_Wallet(Simple_Wallet):
     def delete_address(self, address: str) -> None:
         if not self.db.has_imported_address(address):
             return
+        if len(self.get_addresses()) <= 1:
+            raise Exception("cannot delete last remaining address from wallet")
         transactions_to_remove = set()  # only referred to by this address
         transactions_new = set()  # txs that are not only referred to by address
         with self.lock: