Fix Type Conversion Warning

• Fixes an issue with the SDK that would print a warning for the private _handleMethod() method.
• It was returning the results of calling private handler references, which themselves returned void. However the function is defined to return Future<Null> not void, so a warning was printed.
• Fixed by returning null at the end. The result of this function is not used and the SDK does not need to wait asynchronously for the handlers to finish executing.

Author: Nightsd01

lib/onesignal.dart

@@ -266,23 +266,23 @@ class OneSignal {
   Future<Null> _handleMethod(MethodCall call) async {
     if (call.method == 'OneSignal#handleReceivedNotification' &&
         this._onReceivedNotification != null) {
-      return this._onReceivedNotification(
+      this._onReceivedNotification(
           OSNotification(call.arguments.cast<String, dynamic>()));
     } else if (call.method == 'OneSignal#handleOpenedNotification' &&
         this._onOpenedNotification != null) {
-      return this._onOpenedNotification(
+      this._onOpenedNotification(
           OSNotificationOpenedResult(call.arguments.cast<String, dynamic>()));
     } else if (call.method == 'OneSignal#subscriptionChanged' &&
         this._onSubscriptionChangedHandler != null) {
-      return this._onSubscriptionChangedHandler(
+      this._onSubscriptionChangedHandler(
           OSSubscriptionStateChanges(call.arguments.cast<String, dynamic>()));
     } else if (call.method == 'OneSignal#permissionChanged' &&
         this._onPermissionChangedHandler != null) {
-      return this._onPermissionChangedHandler(
+      this._onPermissionChangedHandler(
           OSPermissionStateChanges(call.arguments.cast<String, dynamic>()));
     } else if (call.method == 'OneSignal#emailSubscriptionChanged' &&
         this._onEmailSubscriptionChangedHandler != null) {
-      return this._onEmailSubscriptionChangedHandler(
+      this._onEmailSubscriptionChangedHandler(
           OSEmailSubscriptionStateChanges(
               call.arguments.cast<String, dynamic>()));
     }