在控制器中调用私有方法返回null值

I'm trying to delete a post but whenever I delete a post its tag is not deleted. I'm using breakpoint to see how things happening and I noticed that GetPost doesn't include the tag whenever it is called. In my codes below you can see that i'm trying to insert/call tagg in the delete method but seems cannot call it.I'm trying to delete a post but whenever I dele