Repository: flutter
Issue: CupertinoTextField  in CupertinoNavigationBar with set focus node does not show keyboard after navigating back
Labels: a: text input,framework,f: cupertino,f: focus,has reproducible steps,found in release: 2.0,found in release: 2.3,team-design,triaged-design

Happened on both iOS and Android. Problem only happens if the focus node is set.
## Steps to Reproduce

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Run `flutter create bug`.
2. Update the files as follows: replace main.dart with:
```
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CupertinoApp(
      home: SearchPage(),
    );
  }
}

class SearchPage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _SearchPageState();
}

class _SearchPageState extends State<SearchPage> {
  final _searchFocusNode = FocusNode();

  @override
  Widget build(BuildContext context) {
    return CupertinoPageScaffold(
      navigationBar: CupertinoNavigationBar(
        middle: CupertinoTextField(
          onSubmitted: (_) => _showDummyPage(context),
          focusNode: _searchFocusNode,
        ),
      ),
      child: SizedBox.shrink(),
    );
  }

  void _showDummyPage(final BuildContext context) {
    Navigator.of(context).push(
      CupertinoPageRoute(
        builder: (context) {
          return CupertinoPageScaffold(
            child: SizedBox.shrink(),
            navigationBar: CupertinoNavigationBar(),
          );
        },
      ),
    );
  }
}
```
3. Run the app on device
4. Write something in the textfield in the navigation bar
5. Submit the text (which navigates to a new page)
6. Navigate back via the back button in the navigation bar
7. Click into the textfield again

**Expected results:** Keyboard pops up and the user can edit the text in the textfield.

**Actual results:** Keyboard does not appear.

<details>
  <summary>Logs</summary>

```
Output from flutter run --verbose was too long to include and I did not see any problems there.
```

<!--
-->

```
     Analyzing text_field_bug_repro...                                       
No issues found! (ran in 2.1s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
[✓] Flutter (Channel stable, 2.0.6, on macOS 11.2.3 20D91 darwin-x64, locale en-CZ)
    • Flutter version 2.0.6 at /Users/matouskozma/flutter
    • Framework revision 1d9032c7e1 (7 days ago), 2021-04-29 17:37:58 -0700
    • Engine revision 05e680e202
    • Dart version 2.12.3

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/matouskozma/Library/Android/sdk
    • Platform android-30, build-tools 29.0.2
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.5, Build version 12E262
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)

[✓] VS Code (version 1.55.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.22.0

[✓] Connected device (4 available)
    • SM G950F (mobile)          • ce091829a9bb360201                       • android-arm64  • Android 9 (API 28)
    • Matous’s iPhone (mobile)   • 6c023fc6862bdbd0fb6b8d5ebcabf25ba5747202 • ios            • iOS 14.4.2
    • iPhone 12 Pro Max (mobile) • F2FCCF5C-68C1-46EF-A459-FB08F8C7092B     • ios            • com.apple.CoreSimulator.SimRuntime.iOS-14-5 (simulator)
    • Chrome (web)               • chrome                                   • web-javascript • Google Chrome 90.0.4430.93

• No issues found!
```

</details>