Store registered context for sync task unregistrations (#6752)

For issue #6558, this is an attempt at fixing the
IllegalArgumentException by ensuring that the context we use for
registering the SyncTask is the same context we use to unregister the
task. Race conditions dont seem like a culprit here since unregister is
only triggered by the Receiver itself, which should be only executed
synchronously on the main thread.

Author: welishr

firebase-messaging/CHANGELOG.md

@@ -1,4 +1,6 @@
 # Unreleased
+* [changed] Bug fix in SyncTask to always unregister the receiver on the same
+  context on which it was registered.
 
 
 # 24.1.0

firebase-messaging/src/main/java/com/google/firebase/messaging/SyncTask.java

@@ -161,6 +161,7 @@ boolean isDeviceConnected() {
   static class ConnectivityChangeReceiver extends BroadcastReceiver {
 
     @Nullable private SyncTask task; // task is set to null after it has been fired.
+    @Nullable private Context receiverContext;
 
     public ConnectivityChangeReceiver(SyncTask task) {
       this.task = task;
@@ -171,7 +172,10 @@ public void registerReceiver() {
         Log.d(TAG, "Connectivity change received registered");
       }
       IntentFilter intentFilter = new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION);
-      task.getContext().registerReceiver(this, intentFilter);
+      if (task != null) {
+        receiverContext = task.getContext();
+        receiverContext.registerReceiver(this, intentFilter);
+      }
     }
 
     @Override
@@ -191,7 +195,9 @@ public void onReceive(Context context, Intent intent) {
         Log.d(TAG, "Connectivity changed. Starting background sync.");
       }
       task.firebaseMessaging.enqueueTaskWithDelaySeconds(task, 0);
-      task.getContext().unregisterReceiver(this);
+      if (receiverContext != null) {
+        receiverContext.unregisterReceiver(this);
+      }
       task = null;
     }
   }