Fix crashing when typing in show only people list
Resolves crashing when typing a filter into the show only people list text field. As soon as a invalid filter was entered, e.g., "1-" from "1-5" PeTrack crashed.
Now when a invalid input is observed, the border will be marked as red and an empty set will be returned. When the input becomes valid again, the border color is restored and the parsed input returned.
Closes #241 (closed)