File "electrum/gui/qt/main_window.py", line 3256, in closeEvent
  File "electrum/gui/qt/main_window.py", line 3277, in clean_up
  File "electrum/gui/qt/__init__.py", line 307, in close_window
  File "electrum/daemon.py", line 286, in stop_wallet
  File "electrum/wallet.py", line 240, in stop_threads
  File "electrum/util.py", line 356, in <lambda>
  File "electrum/util.py", line 352, in do_profile
  File "electrum/storage.py", line 89, in write
  File "electrum/storage.py", line 98, in _write
  File "electrum/json_db.py", line 86, in wrapper
  File "electrum/json_db.py", line 120, in dump
  File "json/__init__.py", line 238, in dumps
  File "json/encoder.py", line 201, in encode
  File "json/encoder.py", line 430, in _iterencode
  File "json/encoder.py", line 404, in _iterencode_dict
  File "json/encoder.py", line 353, in _iterencode_dict