I've just skimmed through the articles, and it isn't obvious
However, I think the clue may lie on page 84 of LXF58, at the start of the "Beginning to code" paragraph.
It says "Before we go any further, ensure that you give the bottom list box widget a name in QT Designer". Taking that in conjunction with the comments on page 86, where the picList->Insert command is discussed, I would hazard a guess that you are supposed to have given the bottom list box widget the name "picList"...
Also note the Top Tip on page 97 of LXF59 about doing a Build->Distclean.