feat(flutter): add troubleshoot for isolates (#13218)

* Update troubleshooting.mdx

* Apply suggestions from code review

Co-authored-by: Alex Krawiec <[email protected]>

---------

Co-authored-by: Alex Krawiec <[email protected]>

Author: 2 people

docs/platforms/dart/guides/flutter/troubleshooting.mdx

@@ -37,6 +37,10 @@ Building a Flutter app for a specific ABI using the `--target-platform` argument
 flutter build apk --target-platform=android-arm --split-per-abi
 ```
 
+## Running Sentry Within an Isolate
+
+If you have a Sentry instance running within a separate [Isolate](https://dart.dev/language/isolates) you must execute `Sentry.close()` before the Isolate completes, otherwise the Isolate won't shut down correctly.
+
 ## Native Symbolication on Android
 
 The configuration for symbolication of Native events (C/C++) is documented in our [Android Native Development Kit](/platforms/android/configuration/using-ndk/) content. If you are having issues with symbolication in Flutter, check that your configuration is correct, as discussed in our Flutter content that covers [Uploading for Android NDK](/platforms/dart/guides/flutter/upload-debug/#uploading-for-android-ndk)