Sharing to Custom Activity causes BAD_ACCESS

[rlustemberg]

When the completion callback is executed, and the 'complete' event is fired, [self ACTIVITY_CUSTOM] is passed as the activity key to the event before being instantiated.

 NSString *activity = activityType;
      if ([activityType rangeOfString:@"com.apple.UIKit.activity"].location == NSNotFound) {
        activity = [self ACTIVITY_CUSTOM];
      }

      NSDictionary *event = @{
        @"success" : @YES,
        @"platform" : @"activityView",
        @"activity" : activity,
        @"activityName" : activityType
      };
      [self fireEvent:@"complete" withObject:event];

I'll provide a PR