删除链表中的节点无效?

I am using templates in c++ to create a linked list. I have coded a few functions , but the deletion of node ( either from head/tail ) isnt working properly. It displays an error and terminates. When debugging , the 'this' shown in watch windows contains a head which is null, which i have no idea why. Here is my code:I am using templates in c++ to create a linked