MJLevelOrderPrinter 删除所有的null时有bug

问题代码:

[rowNodes removeObject:[NSNull null]];

可以修改为

NSMutableArray *array = rowNodes.mutableCopy;
        for (id data in array) {
            if ([data isKindOfClass:[NSNull class]]) {
                [rowNodes removeObject:data];
            }
        }