Task duration below zero

[roman-g-levin]

At a moment GanttProject refused to open the project and froze.
I looked for the error by manually deleting tasks in the .gan file.
The program freeze was caused by a task with a parameter duration = "-4", which has subtasks:

<task id="1628" name="MyTaskName" color="#000000" meeting="false" start="2024-03-11" duration="-4" complete="0" thirdDate="2024-01-07" thirdDate-constraint="0" expand="false">

I have no understanding or ideas how the duration parameter became negative.