Unintentional switch to night mode after starting calibration

I noticed the following behaviour multiple times, but I cannot reproduce it reliably.

After pressing the "Flash" button a long time to initiate calibration and releasing it after the finished countdown, the ampel switches to night mode.

AFAICT this happens if the button is still pressed during the next call of checkFlashButton() and released < 300 ms after that.

IMHO the most reliable fix would be to disable the button polling during calibration. More simple but unreliable would be to introduce a small delay after calling sensor::startCalibrationProcess().