In kivy, if the user enabled fx rates but did not touch the fx history settings,
the GUI would show that history rates are enabled but in fact they would be disabled:
the GUI called fx.get_history_config(default=True) when displaying the checkbox,
but exchange_rate.py would not fetch history rates.
(it would only get fixed if the user touched the fx history checkbox)
Note: FxThread.run() calls fx.show_history(), which calls fx.get_history_config() without arguments.
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