Unuseful error mesage with missing API key

[merydian]

(Prio) When starting fresh without an API Key creating points with live preview fails. Of course. But it should be caught with showing a sensible message to the user.

See #314

[merydian]

@MichaelsJP could you reconfirm this issue persists? On my end I get a QMessagebox stating that there is no API key set after digitization of more than one point.