Program received signal SIGSEGV, Segmentation fault where is the problem the pointer is not null

#include <iostream>
#include<string.h>

using namespace std;

//Class Node Linked List

class Node
{
public:
  string data;
  Node *next;
};

//Stack class

class stack
{
public:
  Node * headref = new Node ();
  //top = new Node();

  //POP ELEMENT FROM STACK
  string pop ()
  {
    string top;
    if (headref == NULL)
      {
    cout << "Stack is empty";
    return "EMPTY";
      }
    else
      {
    top = headref->data;
    headref = headref->next;
    return top;
      }

    return top;
  }

//PUSH ELEMENT IN STACK

  string push (string element)
  {
    if (headref == NULL)
      {
    headref->data = element;
    headref->next = NULL;
      }
    else
      {
    Node *newnode = new Node ();
    newnode->data = element;
    newnode->next = headref;
    headref = newnode;
      }
    return "Hello";
  }

  //RETURNS TOP OF STACK
  string top()
  {
//Segmentation Fault right here

      string temp = headref->data;
    return temp;
  }
  //print stack
  void printStack ()
  {
    while (headref != NULL)
      {
    cout << headref->data << "\n";
    headref = headref->next;
      }
  }
};

//Converts infix to Postfix

class convert:public stack
{
private:
  int precidence (string operatoor)
  {
    if (operatoor == "*" || operatoor == "/")
      {
    return 2;
      }
    if (operatoor == "+" || operatoor == "-")
      {
    return 1;
      }
    if (operatoor == "^")
      {
    return 0;
      }
  }
public:
  string algorithmInfix (string infix)
  {
    string postfix;
    for (int i = 0; i < infix.length (); i++)
      {
    string temp;
    temp.push_back (infix[i]);
    if (isalpha (infix[i]) || isdigit (infix[i]))
      {
        postfix = postfix + infix[i];
      }
    else
      {
        if (temp == "(")
          {
        cout << "( goes in stack";
        push (temp);
        continue;
          }
        else if (temp == ")")
          {
        cout << "Comming out of stack";
        while (top () != "(")
          {
            postfix = postfix + pop ();
          }
        pop ();
          }
        if (top () == "" || top () == "("
        || (precidence (temp) >= precidence (top ())))
          {
        push (temp);
          }
        else
          {
        while (precidence (top ()) >= precidence (temp))
          {
            if (top () == "(" || top () == ")")
              {
            push (temp);
            break;
              }
            postfix = postfix + pop ();
          }
        push (temp);
          }
      }
      }
    string temp = top ();
    while (true)
      {
    if (temp == "")
      {
        break;
      }
    postfix = postfix + pop ();
    temp = top ();
      }
    /*while(top())
       postfix = postfix + pop(); */
    return postfix;
  }
};

//Main

int main ()
{
  //stack obj;
  convert obj;
  string infix;
  while (true)
    {
      cout << "Insert your Infix statement. To Exit type exit!!:";
      //cin.ignore();
      getline (cin, infix);
      if (infix == "exit" || infix == "EXIT" || infix == "Exit")
    {
      break;
    }
      //cout<<"here";
      string postfix;
      postfix = obj.algorithmInfix (infix);
      //cout<<"here";
      cout << infix << " " << postfix << endl;
    }
  /*obj.push('5');
     obj.push('6');
     string top = obj.pop();
     cout<<"Poped this element->"<<top<<endl;
     obj.push('*'); */
  obj.printStack ();
  //cout<<"Hello"<<endl;
}