Why do I keep on getting these access violations?

I am making a priorityQueue with a linkedList, I understand everything except for my extractMin(), I basically made a copy of the original pQueue and traversed through the copy to find the smallest data field and placed a pointer on that node and compared to the rest of the nodes, then once found, return the smallest data

this it the pritorityQueueLL.h

#include <iostream>
#include <cmath>
using namespace std;

class priorityQueueLL
{
private:

    class node
    {
    public:
        //put what you need here..
        node *prev, *next;
        int data;
        node(int x)
        {
            data = x;
            prev = NULL;
            next = NULL;
        }
    };  

    node * top;
    node * bottom;

public:

    priorityQueueLL()
    {
        top = NULL;
        bottom = NULL;

    }

    ~priorityQueueLL()
    {}

    //return true if empty, false if not
    bool empty()
    {
        if(top == NULL)
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    //add item
    void insert(int x)
    {
        node * ai = new node(x);
        if(top == NULL)
        {
            top = ai;
            bottom = ai;
        }
        else
        {
            bottom -> next = ai;
            ai -> prev = bottom;
            bottom = ai;
        }
    }

    //remove and return smallest item
    int extractMin()
    {
        node * copy = top;
        node * end = bottom;
        int val;
        //if there is one node in the pq
        if(top -> next == NULL)
        {
            node * tmp = top;
            top = NULL;
            return tmp -> data;
            delete tmp;
        }
        //if it is empty, return out
        if( empty() )
        {
            return 0;
        }
        else //if it has at least two nodes in the pq
        {
            node * smallest = copy;
            node * tmp = copy;
            while(tmp != NULL)
            {
                val = smallest -> data;
                tmp = tmp -> next;
                if(tmp -> data < val)
                {
                    val = tmp -> data;
                    smallest = tmp;
                    return val;
                    tmp -> prev -> next = tmp -> next;
                    tmp -> next -> prev = tmp -> prev;
                    delete tmp;


                }

            }


        }
    }

};

this is main.cpp

#include <iostream>
#include "stackLL.h"
#include "queueLL.h"
#include "priorityQueueLL.h"
using namespace std;

int main()
{
    /////////////Test code for stack ///////////////
    stackLL stk;

    stk.push(5);
    stk.push(13);
    stk.push(7);
    stk.push(3);
    stk.push(2);
    stk.push(11);

    cout << "Popping: " << stk.pop() << endl;
    cout << "Popping: " << stk.pop() << endl;

    stk.push(17);
    stk.push(19);
    stk.push(23);

    while( ! stk.empty() )
    {
        cout << "Popping: " << stk.pop() << endl;
    }

    // output order: 11,2,23,19,17,3,7,13,5

    ///////////////////////////////////////

    ////////////Test code for queue ///////////

    queueLL Q;

    Q.enqueue(1);
    Q.enqueue(2);
    Q.enqueue(3);
    cout << "Dequeuing: " << Q.dequeue() << endl;
    cout << "Dequeuing: " << Q.dequeue() << endl;
    Q.enqueue(4);
    Q.enqueue(5);

    while( ! Q.empty() )
    {
        cout << "Dequeuing: " << Q.dequeue() << endl;
    }

    ///////////////////////////////////////////



    ////////////Test code for priority queue/////

    priorityQueueLL pQueue;

    const int SIZE = 20;

    //insert a bunch of random numbers
    for(int i=0; i<SIZE; i++)
    {
        pQueue.insert( rand() );
    }

    //pull them back out..
    while( ! pQueue.empty() )
    {
        cout << pQueue.extractMin() << endl;
    }

    ///////////////////////////////////////////

    system("pause");
    return 0;
}

any ideas on what I could be doing wrong?