Traceback (most recent call last):
  File "electrum\util.py", line 1063, in run_with_except_hook
  File "threading.py", line 870, in run
  File "electrum\gui\qt\main_window.py", line 2960, in privkeys_thread
  File "electrum\wallet.py", line 573, in export_private_key
  File "electrum\keystore.py", line 616, in get_private_key
  File "electrum\keystore.py", line 569, in get_master_private_key
  File "electrum\crypto.py", line 290, in pw_decode
  File "electrum\crypto.py", line 249, in pw_decode_bytes
  File "base64.py", line 80, in b64decode
  File "base64.py", line 45, in _bytes_from_decode_data
TypeError: argument should be a bytes-like object or ASCII string, not 'NoneType'