Fix Flutter MissingPluginException on iOS after upgrade

Happens when an iOS app can't find a plugin's native method after updating dependencies. The culprit is almost always a stale Podfile.lock or a missing plugin registration.

When This Error Hits

You just upgraded your Flutter dependencies — maybe you ran flutter pub upgrade or bumped a plugin version in pubspec.yaml. Then you build for iOS, run the app, and boom: MissingPluginException when you try to call any platform-specific method (like camera, location, or file picker). The worst part? Android works fine. iOS crashes with a vague error saying the plugin's method channel isn't found.

I've seen this exact scenario dozens of times. It's almost always a mismatch between what Flutter expects and what the iOS native side has registered. The upgrade broke the bridge.

Root Cause (Plain English)

Flutter plugins work through method channels — a Dart side calls a native method on iOS (or Android). When you upgrade a plugin, the Dart side gets the new channel name or method signature. But the iOS side still has the old registration from the previous Podfile.lock and compiled binaries. The new AppDelegate.swift (or Swift AppDelegate in newer Flutter) doesn't include the plugin's generated registration because the cache is stale.

In Flutter 3.x and later, plugins are auto-registered via FlutterPluginRegistrant. If that file is outdated — which happens when you upgrade without a full clean — the native side doesn't know about the plugin. Result: MissingPluginException.

The Fix — Numbered Steps

Clean everything. Run these commands in your project root:
```
flutter clean
cd ios
rm -rf Podfile.lock
pod deintegrate
pod cache clean --all
cd ..
flutter pub get
```
This removes all cached iOS dependency artifacts. Don't skip pod deintegrate — it removes the Podfile's integration from the Xcode project, forcing a fresh setup.
Reinstall iOS pods. Then run:
```
cd ios
pod install --repo-update
```
The --repo-update flag forces CocoaPods to refresh its local specs repo. Without it, you might get an older version cached. This step can take a minute.
Rebuild the app. Back in the project root:
```
flutter run
```
This regenerates the FlutterPluginRegistrant in ios/Runner/GeneratedPluginRegistrant.swift (or .m if using Objective-C). Open that file after the build — check that your upgraded plugin's class is listed there. If it's missing, something went wrong in step 2.
If that fails, delete DerivedData. Xcode caches old build artifacts there. Run:
```
rm -rf ~/Library/Developer/Xcode/DerivedData
```
Then rebuild from scratch with flutter run. This is nuclear but works when the cache is stubborn.
Check your AppDelegate. In Flutter 3.x+, you should not manually register plugins in AppDelegate.swift. The generated registrant handles it. If you have custom code like GeneratedPluginRegistrant.register(with: self), comment it out. Let Flutter's auto-registration do its job. Duplicate registration causes silent failures.

What to Check If It Still Fails

Plugin compatibility. Some plugins don't support the latest Flutter version yet. Check the plugin's CHANGELOG or GitHub issues. I've had this with older versions of url_launcher and image_picker.
iOS minimum deployment target. If a newer plugin requires iOS 14+ but your project targets iOS 12, the native code won't compile. Set it in ios/Podfile to platform :ios, '14.0' (or whatever the plugin needs).
Flutter version. Run flutter --version. If you're on a very old channel (like 2.x), upgrade to at least 3.10. The auto-registration logic changed significantly after 3.0.
Multiple plugins with same channel name. Rare, but possible if you forked a plugin. Check that no two plugins use the same method channel name. MissingPluginException can also mean the channel name is already taken by another plugin that didn't register properly.

If you've done all that and it still breaks, open the Xcode project (ios/Runner.xcworkspace) and build from there. Look for build warnings about missing plugin symbols. Usually it's a pod version mismatch — pin the plugin version in pubspec.yaml to the one that worked before the upgrade, then try a different version.