Steps to reproduce:
1. Place a pickup skill, entrance or exit
2. Edit the skill count or lemming count by typing in a value (not by using the up/down clickable buttons or arrow keys)
3. Click in the level area such that the pickup skill / entrance / exit is de-selected
The change does not get applied, and an error message pops up.
Workaround: Click somewhere that doesn't deselect the object first. This could be another edit field (like the skill checkboxes), or clicking in the level area somewhere that doesn't deselect the object.
Dullstar's topic here references what I believe to be specific cases of the same issue: https://www.lemmingsforums.net/index.php?topic=4363.0
Fixed, I think, in commit 85A1DDB.