As visible in the red rectangle down, my worker skill change is failed and I got an error, despite as visible on the image I clearly selected a skill.

The cause of this bug is, that there is a another widget bellow the skill change widget, and as I clicked to the area in the red rectangle it somehow not registered it as selection.

If I clik to a another point, where there is no other widget bellow the skill change widget, it works fine.