一个简单的成员函数使用成员变量的例子，出点小疑点

#include<iostream>

using namespace std;

class Linked{
      protected:
                struct Node{
                       int item;
                       Node *next;
                       };
                Node* head;
                long length;
      public:

             Linked(){length=0;}
             void insert(int i){
                              if((head->next)==NULL){
                                      Node* newItem;
                                      newItem->item=i;
                                      head->next=newItem;
                                      length++;
                                      cout<<this->length;
                              }
                  }
      };
int main()
{
    Linked itr;
    itr.insert(2);
    system("pause");
    return 0;
}

Unhandled exception at 0x004114c8 in test.exe: 0xC0000005: Access violation reading location 0xccccccd0.
First-chance exception at 0x004114c8 in test.exe: 0xC0000005: Access violation reading location 0xccccccd0.