EMailConfigFormatHTML listens for "changed" signals from a GSettings
instance, passing itself as the 'user_data' to the signal handler. But
the signal handler was left connected after EMailConfigFormatHTML was
finalized, resulting in the signal handler receiving a dangling pointer.
Not using g_signal_connect_object() here because of the unresolved
reference leak issue in GObject. The GSettings instance is likely
cached internally and lives well beyond the EMailConfigFormatHTML.
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