project.exe has triggered a breakpoint - c++

Question

I had a circular link list which has add, delete, printlist and search functions etc. All the functions works correctly but when I delete all nodes in the list and than try to add new nodes to the same list, I get project.exe has triggered a breakpoint message.
List.h
#ifndef LIST_H
#define LIST_H
using namespace std;
class list {
public:
struct node {
string data, assignee, date;
node *next;
};
typedef struct node NodePtr;
NodePtr *head, *current, *temp, *dummy;
list();
void add();
void del();
void printList();
void menu();
void exit();
void search();
};
#endif
Source.cpp
#include <cstdlib>
#include <iostream>
#include "List.h"
#include<string>
using namespace std;
list::list() { //constructor
head = NULL;
current = NULL;
temp = NULL;
}
void list::add(){
string addTask, addAssignee, addDate;
NodePtr *newNode = new NodePtr;
//entering the data of the tasks
cout << "Enter the name of the task:\n"<<"<<";
getline(cin, addTask); // compailer doesnt detect the first getline function, ı dont understan why. ı had to put it twice
getline(cin, addTask);
cout << "Enter the assignee name for the task:\n" << "<<";
getline(cin, addAssignee);
cout << "Enter the deadline for the task:\n" << "<<";
getline(cin, addDate);
cout << "\n";
//assignt data to the new node
newNode->data = addTask;
newNode->assignee = addAssignee;
newNode->date = addDate;
newNode->next = NULL;
if (head != NULL){
current = head;
while (current->next != head){ //finding the last node
current = current->next;
}
current->next = newNode;
dummy = newNode;
dummy->next = head; // we make the list, circular
}
else //here will work if the list is empty
{
head = newNode;
dummy = newNode;
head->next=head;
}
}
void list::printList(){
int i = 1;
current = head;
if (head == NULL){
cout << "List is empty\n\n";
}
else
{
do
{
cout << i << "-";
cout << current->data << endl;
cout << current->assignee << endl;
cout << current->date << endl << endl;
current = current->next;
i++;
} while (current != dummy->next);
}
}
void list::del(){ //tüm liste silindikten sonra tekrar işlem(ekleme, listeleme etc.) yapılamıyor hata veriyor
NodePtr *delPtr;
delPtr = NULL;
int ID;
cout << "These tasks exist in the list:\n";
printList();
cout << "Choose the ID of the task you want to remove:\n";
cin >> ID;
if (head == NULL){
cout << "List does not includes any task.\n";
}
else
if (ID == 1){
delPtr = head;
head = head->next;
delete delPtr;
dummy->next = head;
}
else
{
current = head;
for (int k = 1; k <= ID - 1; k++){
temp = current;
current = current->next;
}
if (current == dummy){
dummy = temp;
dummy->next = head;
}
delPtr = current;
current = current->next;
temp->next = current;
delete delPtr;
}
}
void list::menu(){
cout << "Choose an operation\n"
<< "A: Add Task\n"
<< "S: Search for Task\n"
<< "L: List All Tasks\n"
<< "R: Remove Task\n"
<< "E: Exit\n\n"<<"<<";
}
void list::search(){
string searchTask;
NodePtr *search;
search = head;
cout << "To search for a task, enter its task name or assignee name:\n" << "<<";
getline(cin, searchTask);
getline(cin, searchTask);
while (search != dummy){//bu satır problemli gibi gözüküyor
if (search->data == searchTask || search->assignee == searchTask){
cout << search->data << endl
<< search->assignee << endl
<< search->date << endl<<endl;
}
search = search->next;
}
}
void list::exit(){
node *delPtr;
current = head;
if (head != NULL){
do{
temp = current;
delPtr = temp;
current = current->next;
delete delPtr;
} while (current != dummy);
}
}
Main.cpp
#include <cstdlib>
#include <iostream>
#include "List.h"
#include<string>
using namespace std;
int main(){
char operation;
list tasks;
tasks.menu();
cin >> operation;
while (operation!='E' && operation!='e'){
switch (operation)
{
case 'A':case'a':
tasks.add();
break;
case 'S': case's':
tasks.search();
break;
case 'L': case 'l':
cout << "All tasks are listed below:\n";
tasks.printList();
break;
case 'R': case 'r':
tasks.del();
break;
default:
break;
}
tasks.menu();
cin >> operation;
}
tasks.exit();
return 0;
}