Why can't I access the rebuild() method from the state object?

dice_row.dart

import 'package:flutter/material.dart';
import 'dice_button.dart';
import 'dice_rack.dart';
import 'dart:developer';

class DiceRow extends StatefulWidget {
  final _RowState content = _RowState();
  final RackState rack;

  DiceRow(this.rack);

  bool empty() => content.empty();

  void rebuild() {
    rack.rebuild();
  }

  @override
  _RowState createState() => content;
}

class _RowState<DiceRow> extends State {
  List<DieWidget> _dice = [];

  bool empty() => _dice.length == 0;

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 80,
      decoration: BoxDecoration(
        border: Border.all(
            color: Colors.green,
            width: 5.0,
            style: BorderStyle.solid
        ),
      ),
      child: _rowContent(),
    );
  }

  _rowContent([Widget dropped]) {
    if (dropped != null)
      _dice.add(dropped);

    return DragTarget<DiceDragInfo>(
      onWillAccept: (data) => true,
      onAccept: (data) {
        setState(() => _dice.add(
            DieWidget(
              data.sides,
              data.displayAs,
              DraggableDieState(
                data.sides,
                data.displayAs
              )
            )
        ));
        debugger();
        widget.rebuild();
      },
      builder: (context, data, rejected_data) {
        return Row(
          mainAxisSize: MainAxisSize.max,
          children: [
            IconButton(
              icon: Icon(Icons.refresh),
              tooltip: 'Reroll',
              onPressed: _rollAll,
            ),
            ListView(
              shrinkWrap: true,
              children: _dice + [],
              scrollDirection: Axis.horizontal,
            )
          ]
        );
      }
    );
  }

  _rollAll() {
    setState(() {
      _dice.forEach((die) {
        die.recalc();
      });
    });
  }
}