[iOS][Text Input] Avoid crash when its UIViewController.view is nil.

[luckysmg]

Fix:

• iOS 15 crash: The application must have a host view since the keyboard client must be part of the responder chain to function. The host view controller is (null) flutter#106404

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[LongCatIsLooong]

Could you help me understand when this could happen? From the stacktrace here: flutter/flutter#106404 (comment), not handling

-[FlutterTextInputPlugin setTextInputClient:withConfiguration:]

could be disastrous (since the framework always assumes the connection is successfully established), unless the engine is about to be torn down. But if the engine is being deallocated we're not supposed to get any platform messages right?

[luckysmg]

In add-to-app scene. We will share one engine with multiple ViewController, and we will set engine's view controller to nil manually sometimes. Any if some message's coming will trigger this crash....

[luckysmg]

Here is the demo code to reproduce 100% with this crash.
in main.dart:

Dart code

import 'package:flutter/material.dart';

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

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

  // This widget is the root of your application.
  @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> {

  FocusNode node = FocusNode();

  @override
  void initState() {
    super.initState();
    Future.delayed(const Duration(seconds: 5),() {
      node.requestFocus();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'You have pushed the button this many times:',
            ),
            TextField(
              focusNode: node,
            ),
          ],
        ),
      ),
    );
  }
}

in FlutterAppDelegate.swift

swift code

import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    GeneratedPluginRegistrant.register(with: self)
      
      
      let vc = window.rootViewController as! FlutterViewController;
      DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 3){
          let engine = vc.engine
          engine?.viewController = nil
      }
      
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

It is very easy for developers to write code like this (delay some time request focus), if we detach the view controller from engine(This is very common in add-to-app scene), it will cause crash. I think we should do something to fallback, maybe not perfect, but at least, it will be better than crash directly ^_^

[luckysmg]

I have modified the code. I think it's better than before. Plz re-review ^_^