for anyone facing this issue:
NullReferenceException: Object reference not set to an instance of an object
UnitActionSystemUI.CreateUnitActionButtons () (at Assets/Scripts/UnitActionSystemUI.cs:19)
UnitActionSystemUI.Start () (at Assets/Scripts/UnitActionSystemUI.cs:13)
just remove the selected lines in the below image.
& put them in awake, like this