Add null checks, and improve swig error handling

Author: a-maurice

messaging/src/FirebaseNotification.cs

@@ -22,6 +22,8 @@ namespace Firebase.Messaging {
 /// implementation.
 public sealed class AndroidNotificationParams {
   internal static AndroidNotificationParams FromInternal(AndroidNotificationParamsInternal other) {
+    if (other == null) return null;
+
     AndroidNotificationParams android = new AndroidNotificationParams();
     android.ChannelId = other.channel_id;
     return android;
@@ -45,6 +47,8 @@ public void Dispose(bool disposing) { }
 /// library.
 public sealed class FirebaseNotification {
   internal static FirebaseNotification FromInternal(FirebaseNotificationInternal other) {
+    if (other == null) return null;
+
     FirebaseNotification notification = new FirebaseNotification();
     notification.Android = AndroidNotificationParams.FromInternal(other.android);
     notification.Badge = other.badge;

messaging/src/swig/messaging.i

@@ -316,18 +316,22 @@ void SendPendingEvents() {
     // messageReceivedDelegate.
     [MonoPInvokeCallback(typeof(MessageReceivedDelegate))]
     private static int MessageReceivedDelegateMethod(System.IntPtr message) {
+      int tookOwnership = 0;
       return ExceptionAggregator.Wrap(() => {
           // Use a local copy so another thread cannot unset this before we use it.
           var handler = FirebaseMessagingInternal.MessageReceivedInternal;
           if (handler != null) {
+            // Take ownership, and track it so that the caller of this knows, even
+            // if an exception is thrown, since the C# object will still delete it.
             FirebaseMessageInternal messageInternal = new FirebaseMessageInternal(message, true);
+            tookOwnership = 1;
             handler(null, new Firebase.Messaging.MessageReceivedEventArgs(
                 FirebaseMessage.FromInternal(messageInternal)));
             messageInternal.Dispose();
             return 1;
           }
           return 0;
-        }, 0);
+        }, tookOwnership);
     }
 
     // Called from ListenerImpl::TokenReceived() via the