Also make the start checks a bit more robust to broken core code by
double-checking that the palette is valid from the start, even though getting an
invalid palette should normally not happen anymore (at least I fixed one such
case with a palette with a NULL id in my previous commit; but it looks like our
core code is not perfectly robust and we should push the code to extensive
testing).
Note that the generated widget is currently ugly (no label and terrible UX), but
this is meant to improve soon.
f99bdcd008