childCount property Null safety
The total number of children this delegate can provide.
If null, the number of children is determined by the least index for which builder returns null.
May result in an infinite loop or run out of memory if childCount is null
and the builder always provides a zero-size widget (such as
SizedBox.shrink()). If possible, provide children with non-zero size,
return null from builder, or set a childCount.
final int? childCount;