[connectivity] Clear networkCallback object as soon as stream is cancelled (flutter#3303)

Set networkCallback back to null when event stream is cancelled

Author: ened

packages/connectivity/connectivity/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 3.0.0-nullsafety.2
+
+* Android: Cleanup the NetworkCallback object when a connectivity stream is cancelled
+
 ## 3.0.0-nullsafety.1
 
 * Bump Dart SDK to support null safety.

packages/connectivity/connectivity/android/src/main/java/io/flutter/plugins/connectivity/ConnectivityBroadcastReceiver.java

@@ -64,6 +64,7 @@ public void onCancel(Object arguments) {
     if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
       if (networkCallback != null) {
         connectivity.getConnectivityManager().unregisterNetworkCallback(networkCallback);
+        networkCallback = null;
       }
     } else {
       context.unregisterReceiver(this);

packages/connectivity/connectivity/pubspec.yaml

@@ -2,7 +2,7 @@ name: connectivity
 description: Flutter plugin for discovering the state of the network (WiFi &
   mobile/cellular) connectivity on Android and iOS.
 homepage: https://github.com/flutter/plugins/tree/master/packages/connectivity/connectivity
-version: 3.0.0-nullsafety.1
+version: 3.0.0-nullsafety.2
 
 flutter:
   plugin: