File "electrum\gui\qt\history_list.py", line 602, in <lambda>
  File "electrum\gui\qt\main_window.py", line 3458, in bump_fee_dialog
  File "electrum\wallet.py", line 936, in bump_fee
  File "electrum\wallet.py", line 161, in get_locktime_for_new_transaction
  File "electrum\blockchain.py", line 470, in header_at_tip
  File "electrum\blockchain.py", line 186, in func_wrapper
  File "electrum\blockchain.py", line 457, in read_header
  File "electrum\blockchain.py", line 419, in assert_headers_file_available