[share] Don't nullify method channel's call handler when detached from Activity

[mduong]

Description

In an add-to-app Android application, the share method channel is set up with the appropriate MethodCallHandler set when the plugin attaches to the Flutter engine. When the Android app launches a FlutterActivity, onAttachedToActivity is called to set the activity on the Share instance and invoking share works initially. Once the FlutterActivity is dismissed and you're taken back to native, onDetachedFromActivity is executed which nullifies the activity on the Share instance as well as the MethodCallHandler on the method channel. Subsequent launches of FlutterActivity never rebind the MethodCallHandler on the share method channel so any calls to share will then fail with:

Unhandled Exception: MissingPluginException(No implementation found for method share on channel plugins.flutter.io/share)

The solution here is to not nullify the MethodCallHandler when the FlutterActivity is detached and only nullifying the activity on the Share instance.

Related Issues

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[mduong]

/domain @samandmoore @mseijas
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[samandmoore]

oops. fixed. nanda. had forgotten to add proper access.

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[samandmoore]

<<domainlgtm platformlgtm

this looks great. we should open this PR upstream to the flutter plugins repo once we have time ❤️ i'm sure they'd greatly appreciate it

[nanda-prbot]

Approved! 🔩 🎸 🌮