File "/usr/lib/python3/dist-packages/electrum/gui/qt/transaction_dialog.py", line 315, in save
    if self.main_window.save_transaction_into_wallet(self.tx):
  File "/usr/lib/python3/dist-packages/electrum/gui/qt/main_window.py", line 3301, in save_transaction_into_wallet
    if not self.wallet.add_transaction(tx):
  File "/usr/lib/python3/dist-packages/electrum/wallet.py", line 858, in add_transaction
    tx_was_added = super().add_transaction(tx, allow_unrelated=allow_unrelated)
  File "/usr/lib/python3/dist-packages/electrum/address_synchronizer.py", line 263, in add_transaction
    is_coinbase = tx.inputs()[0].is_coinbase_input()