Every time you try to add an item (for example a new skill, spot, or employee) with any missing information (for example an employee without any skill) or completely empty, the following (not exactly user-friendly) error opens:

Suggestions for improvement:

1. A modal window that contains a relevant warning should open instead. For example, It is not possible to add an employee without any skill. Please try again.
2. It should not be possible to click Add if any required information is missing.

The server log is attached.

EDIT: the same error appears when the user tries to add an item with an already existing name.