如何解决这个错误，同时获得的高度的小部件，因此高度的可用工作区域在Flutter

我正在寻找计算两种情况下可用的有效空间:(01)当MIDI键盘不是MIDI，和(02)当MIDI键盘是MIDI.effective space我指的是通过扣除AppBar占用的空间来获得的空间(也许这也会覆盖顶部状态栏？)，然后是下面的TextField，和底部的MIDI键盘(当MIDI)和底部的工具？/导航？wine 吧

我试过问题底部列出的sources个代码.到目前为止，我能够得到设备的物理和逻辑高度，逻辑高度的MIDI键盘，但我卡住了部分—获得的逻辑高度的小部件.我试过实施GlobalKey，但没有成功.我无法得到高度，并且发生了一个错误，我没有成功地解决.错误的位置包含在以下代码的多行注释中.错误说—"属性'size'不能被无条件访问，因为接收器可以是null.... ".我try 使用?和!操作符来解决空条件，但没有任何工作.

如何解决这个问题以获得小部件的高度？此外，对于我的目标提出建议是最受欢迎的.

import 'dart:ui';
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final GlobalKey? _key01 = GlobalKey();

  late final TextEditingController firstController;

  String result = "";
  String effective_available_screen_height = "";
  String dimensions = "";

  FlutterView view = WidgetsBinding.instance.platformDispatcher.views.first;

  late Size size;
  double width = 0;
  double height = 0;
  double height01 = 0;

  @override
  void initState() {
    super.initState();
    firstController = TextEditingController();
  }

  @override
  void dispose() {
    firstController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    Size ph_size = view.physicalSize;
    double ph_height = ph_size.height;

    size = MediaQuery.of(context).size;
    width = size.width;
    height = size.height;

    final padding = MediaQuery.of(context).viewPadding;
    height01 = height - padding.top - padding.bottom;

    effective_available_screen_height = "$height01";

    // Height of Keyboard when onscreen (ohterwise zero):
    double keyboard_height = MediaQuery.of(context).viewInsets.bottom;

    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Container(
        padding: const EdgeInsets.all(5.0),
        child: Column(
          children: [
            Container(
              child: Row(
                children: [
                  Expanded(
                    flex: 5,
                    child: Container(
                      padding: const EdgeInsets.all(5.0),
                      child: TextField(
                        key: _key01,
                        style: const TextStyle(
                          fontSize: 16,
                        ),
                        controller: firstController,
                        keyboardType: TextInputType.number,
                        onChanged: (value) {
                          setState(() {
                            result = value;
                            /*
                            // The following gave error:
                            dimensions =
                                "${_key01?.currentContext.size.height}";
                            */
                            dimensions = "${_key01?.currentContext}";
                          });
                        },
                      ),
                    ),
                  ),
                  const Expanded(
                    flex: 5,
                    child: Text("Input Value"),
                  ),
                ],
              ),
            ),
            Text("Result = $result"),
            Text("Total physical height = $ph_height"),
            Text("Total logical height = $height"),
            Text("Onscreen Keyboard height = $keyboard_height"),
            Text(
                "Working Height Available (logical) = $effective_available_screen_height"),
            Text("Dimensions: $dimensions"),
          ],
        ),
      ),
    );
  }
}

Sources of Help:

• How to Determine Screen Height and Width
• Get keyboard height in Flutter