contextを引数として貰えるbuild()内やライフサイクルメソッド内で、別画面への画面遷移を行う方法

Question

ImagePicker.getImage()にて実装したカメラ機能で、カメラ撮影後「OK」を押下すると
別の画面遷移としての処理である、Navigator.pushでこけて失敗します。
以下、エラーメッセージです。

I/Timeline( 6877): Timeline: Activity_idle id: android.os.BinderProxy@ede736a time:6959504
E/flutter ( 6877): [ERROR:flutter/lib/ui/ui_dart_state.cc(166)] Unhandled Exception: NoSuchMethodError: The method 'findAncestorStateOfType' was called on null.
E/flutter ( 6877): Receiver: null
E/flutter ( 6877): Tried calling: findAncestorStateOfType<NavigatorState>()
E/flutter ( 6877): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
E/flutter ( 6877): #1      Navigator.of (package:flutter/src/widgets/navigator.dart:2136:19)
E/flutter ( 6877): #2      Navigator.push (package:flutter/src/widgets/navigator.dart:1758:22)
E/flutter ( 6877): #3      CameraState.transitionTo (package:flutter_gowjp/camera.dart:127:15)
E/flutter ( 6877): #4      CameraState._takePicture.<anonymous closure> (package:flutter_gowjp/camera.dart:95:9)
E/flutter ( 6877): #5      State.setState (package:flutter/src/widgets/framework.dart:1236:30)
E/flutter ( 6877): #6      CameraState._takePicture (package:flutter_gowjp/camera.dart:93:7)
E/flutter ( 6877): <asynchronous suspension>
E/flutter ( 6877): #7      CameraState.initState (package:flutter_gowjp/camera.dart:110:5)
E/flutter ( 6877): #8      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4649:58)
E/flutter ( 6877): #9      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4485:5)
E/flutter ( 6877): #10     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3455:14)
E/flutter ( 6877): #11     Element.updateChild (package:flutter/src/widgets/framework.dart:3223:18)
E/flutter ( 6877): #12     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5839:14)
E/flutter ( 6877): #13     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3455:14)
E/flutter ( 6877): #14     Element.updateChild (package:flutter/src/widgets/framework.dart:3223:18)
E/flutter ( 6877): #15     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4536:16)
E/flutter ( 6877): #16     Element.rebuild (package:flutter/src/widgets/framework.dart:4227:5)
E/flutter ( 6877): #17     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4490:5)
E/flutter ( 6877): #18     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4485:5)
E/flutter ( 6877): #19     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3455:14)
E/flutter ( 6877): #20     Element.updateChild (package:flutter/src/widgets/framework.dart:3223:18)
E/flutter ( 6877): #21     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5839:14)
E/flutter ( 6877): #22     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3455:14)
E/flutter ( 6877): #23     Element.updateChild (package:flutter/src/widgets/framework.dart:3223:18)
E/flutter ( 6877): #24     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5839:14)
E/flutter ( 6877): #25     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3455:14)
E/flutter ( 6877): #26     Element.updateChild (package:flutter/src/widgets/framework.dart:3223:18)
E/flutter ( 6877): #27     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4536:16)
E/flutter ( 6877): #28     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4684:11)
E/flutter ( 6877): #29     Element.rebuild (package:flutter/src/widgets/framework.dart:4227:5)
E/flutter ( 6877): #30     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4490:5)
E/flutter ( 6877): #31     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4675:11)
E/flutter ( 6877): #32     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4485:5)
E/flutter ( 6877): #33     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3455:14)
E/flutter ( 6877): #34     Element.updateChild (package:flutter/src/widgets/framework.dart:3223:18)
E/flutter ( 6877): #35     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5839:14)
E/flutter ( 6877): #36     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3455:14)
E/flutter ( 6877): #37     Element.updateChild (package:flutter/src/widgets/framework.dart:3223:18)
E/flutter ( 6877): #38     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5839:14)
E/flutter ( 6877): #39     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3455:14)
E/flutter ( 6877): #40     Element.updateChild (package:flutter/src/widgets/framework.dart:3223:18)
E/flutter ( 6877): #41     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4536:16)
E/flutter ( 6877): #42     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4684:11)
E/flutter ( 6877): #43     Element.rebuild (package:flutter/src/widgets/framework.dart:4227:5)
E/flutter ( 6877): #44     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4490:5)
E/flutter ( 6877): #45     StatefulElement._fir

navigator.dart:2136の中を見ると、上記のメソッドはContextのメソッドなので
Navigator.push()をする処理が早く、Contextが良い状態になってないのかなと予想しています。
そのため、Contextが引数として貰える別の場所にて、Navigator.push処理を移動させてみようと考えています。

そこで、現在のcontextを引数として貰えるbuild()内やライフサイクルメソッド内で、
アクション(Navigator.pushの処理)を行う方法をご存知の方がいましたら、ご教授お願いしたく思います。
（ボタンプッシュによるアクション（例えばfloatingactionbuttonのonPressed()）へ処理を移動させることはやりたくありません。）

import 'dart:async';
//ファイル出力用ライブラリ
import 'dart:io';

import 'package:camera/camera.dart';
import 'package:flutter/material.dart';
import 'package:image_picker/image_picker.dart';
import 'package:path_provider/path_provider.dart';
import 'package:path/path.dart' show context, join;

import 'package:flutter/rendering.dart';

import 'package:http/http.dart' as http;

import 'package:logging/logging.dart';
import 'package:fluttertoast/fluttertoast.dart';

import 'package:intl/intl.dart';

import 'package:flutter_gowjp/responseView.dart';
import 'package:permission_handler/permission_handler.dart';

CameraController mController = null;
List<CameraDescription> mCameras = null;
CameraDescription mFirstCamera = null;
Future<void> mInitializeControllerFuture = null;
String mResponseBody = "";
final mLog = new Logger('gowjp(camera)');
String mCashedImageName = "";


class CameraView extends StatefulWidget {

  @override
  State createState() => CameraState();

}

class CameraState extends State<CameraView> {

  final _mPicker = ImagePicker();
  BuildContext mContext = null;

  // カメラを起動する
  void _takePicture() async {

    // ストレージアクセス権限のチェック
    var status = await Permission.storage.status;
    if (!status.isGranted) {
      await Permission.storage.request();
    }
    try {
      final pickedFile = await _mPicker.getImage(source: ImageSource.camera);
      final String nowTime = 'IMG_${DateFormat('yyyyMMddHHmmss').format(DateTime.now())}';
      final String cameraPath = '/storage/emulated/0/DCIM/Camera';

      // "OK"を押すと以下の処理が走る
      // /data/のアプリ領域配下にある撮影画像のパス
      String picFilePath = pickedFile.path;

      setState(() {
        // POST送信するClassへ遷移する
        transitionTo(picFilePath, nowTime);
      });

    } catch (e) {
      print(e);
      Fluttertoast.showToast(msg: "カメラ撮影で予期せぬエラーが発生しました。");
      return;
    }
  }

  @override
  void initState() {
    super.initState();

    // カメラを起動する
    _takePicture();
  }

  @override
  void didChangeDependencies() {
      super.didChangeDependencies();
  }

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

  // POST送信するClassへ遷移する
  void transitionTo(String picFilePath, String picTime) async {
    String dName = 'cameraPicture';

    Navigator.push(
      mContext,
      new MaterialPageRoute<Null>(
        settings: const RouteSettings(name: "/responseView"),
//        builder: (BuildContext context) => ResponseView(picFilePath, dName),
        builder: (BuildContext context) => ResponseView.part2(picFilePath, picTime, dName),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    // 黒画面
    return Scaffold(
        body: Container(
          color: Colors.black,
        )
    );

  }

}

Answer 1

contextを用いない画面遷移の方法があり、以下を参考にして画面遷移ができました。

https://qiita.com/kasa_le/items/8c80ae565186c9ac275a