Linked List_Implementation

Question

I was trying to implement the linked list stl but I always get runtime error if someone can help me solve this problem please. What I was doing is when inserting an element I check if it's the first element if it is I make the head and tail point to it if not I use the node's next pointer to point to the new element and move the tail so it point to the last inserted element the error appears in the insert function.

#include <iostream>
 
using namespace std;
 
template<typename T>
struct Node {
    T item;
    Node *next = nullptr;
};
 
template<typename T>
class list {
    static int sz;
    Node<T> *head, *tail;
public:
    void insert(T x) {
        Node<T> new_node;
        new_node.item = x;
        if (!sz) {
            *head = new_node;
            *tail = new_node;
        } else {
            tail->next = &new_node;
            *tail = new_node;
        }
        sz++;
    }
 
    int size() {
        return sz;
    }
 
    friend ostream &operator<<(ostream &os, list<T> &o) {
        while (o.head->next != nullptr) {
            os << o.head->item << " ";
            o.head = o.head->next;
        }
        return os;
    }
};
 
template<typename T>
int list<T>::sz = 0;
 
int main() {
    list<int> l;
    l.insert(5);
    l.insert(6);
    l.insert(7);
    cout << l.size() << endl;
    cout << l << endl;
    return 0;
}