[ios, osx] Invalidate task immediately upon removal

platform/darwin/include/MGLOfflineStorage.h

@@ -70,8 +70,13 @@ typedef void (^MGLOfflineTaskListingCompletionHandler)(NS_ARRAY_OF(MGLOfflineTas
 
 /**
  Unregisters the given offline task and frees any resources that are no longer
- required by any remaining tasks. After this task is removed, it is invalid; any
- attempt to interact with it will result in an exception being thrown.
+ required by any remaining tasks.
+ 
+ As soon as this method is called on a task, the task becomes invalid; any
+ attempt to send it a message will result in an exception being thrown. If an
+ error occurs and the task cannot be removed, do not attempt to reuse the task
+ object. Instead, use the `-getTasksWithCompletionHandler:` method to obtain a
+ valid pointer to the task object.
  
  @param task The offline task to remove.
  @param completion The completion handler to call once the task has been

platform/darwin/src/MGLOfflineStorage.mm

@@ -104,6 +104,7 @@
 }
 
 - (void)removeTask:(MGLOfflineTask *)task withCompletionHandler:(MGLOfflineTaskRemovalCompletionHandler)completion {
+    [task invalidate];
     self.mbglFileSource->deleteOfflineRegion(std::move(*task.mbglOfflineRegion), [&, task, completion](std::exception_ptr exception) {
         NSError *error;
         if (exception) {
@@ -113,7 +114,6 @@
         }
         if (completion) {
             dispatch_async(dispatch_get_main_queue(), [&, task, completion, error](void) {
-                [task invalidate];
                 completion(error);
             });
         }