unselectedIconTheme property Null safety
The size, opacity, and color of the icon in the currently unselected BottomNavigationBarItem.icons.
If this is not provided, the size will default to iconSize, the color will default to unselectedItemColor.
It this field is provided, it must contain non-null IconThemeData.size and IconThemeData.color properties. Also, if this field is supplied, selectedIconTheme must be provided.
final IconThemeData? unselectedIconTheme;