File "electrum\gui\qt\network_dialog.py", line 73, in on_update
  File "electrum\gui\qt\network_dialog.py", line 351, in update
  File "electrum\gui\qt\network_dialog.py", line 142, in update
  File "electrum\blockchain.py", line 250, in get_name
  File "electrum\blockchain.py", line 517, in get_hash
  File "electrum\blockchain.py", line 201, in func_wrapper
  File "electrum\blockchain.py", line 473, in read_header
  File "electrum\blockchain.py", line 435, in assert_headers_file_available