children property Null safety
Returns an Iterable of all non-null children.
This getter is used by the default implementation of attach, detach,
redepthChildren, visitChildren, and debugDescribeChildren to iterate
over the children of this
RenderBox. The base implementation makes no
guarantee about the order in which the children are returned. Subclasses,
for which the child order is important should override this getter and
return the children in the desired order.
@protected Iterable<RenderBox> get children => _slotToChild.values;