C＃：帮助确定TreeView中的选定节点索引。 NullReferenceException错误？

I'm trying to make it so I can run certain events when a node is selected in a TreeView. I run the code and I get an error that reads NullReferenceException was unhandled: Object reference not set to an instance of an object.I'm trying to make it so I can run certain even