Fix iOS Background Isolation Error

[SuperMari000]

This PR is related to Issues #233 and #168

[SuperMari000]

@okadan Could you please review this PR?

[obrunsmann]

@SuperMari000 FYI the error message

[ERROR:flutter/shell/common/shell.cc(1064)] The ‘dev.flutter.pigeon.nfc_manager.FlutterApiPigeon.tagSessionDidBecomeActive’ channel sent a message from native to Flutter on a non-platform thread. Platform channel messages must be sent on the platform thread. Failure to do so may result in data loss or crashes, and must be fixed in the plugin or application code creating that channel.

is now gone. However, I still have the problem that as soon as I end/cancel a scan session on iOS without scanning (user canceled), I receive the following message:

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: PlatformException(session_already_exists, Multiple sessions cannot be active at the same time., null, null)

Perhaps this has something to do with the problem?

[lohnn]

@obrunsmann That is not connected to to this PR or issue. That happens when you try to start a new NFC session while the last one is not properly closed.