Tweeten not showing in system tray, but staying active

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Although I've set the settings 'show app on tray' and 'minimize to tray on close', Tweeten doesn't appear in the system tray, but it does stay active. The result is that there is no way to close it. When I then start it again, it opens immediately, but of course it would be better if it remained active in the system tray. OS : Windows 8.1 64-bit