itemExtent property Null safety
final
        The size of the children along the main axis. Children RenderBoxes will be given the BoxConstraints of this exact size.
Must not be null and must be positive.
Implementation
final double itemExtent;
The size of the children along the main axis. Children RenderBoxes will be given the BoxConstraints of this exact size.
Must not be null and must be positive.
final double itemExtent;