Segementation fault in queue program when using push

Im getting a segmentation fault when i try to push elements into the queue, im not an expert working with queues so i dont recognize where the problem is. I have been searching for the solution to this problem and even though people get similar problems i didnt help me fix my problem. Here is the code:

(I used the debug option in Dev-c ++ 5.9.2 and it told me the line "temp->link = NULL;" is causing the problem but i have no idea how to fix it)

#include <iostream>
using namespace std;

struct Node {
    int data;
    Node* link;
};

class Queue {
public:
    Queue(); 
    ~Queue(); 
    void pushBack(int d);
    bool popFront(); 
    bool isEmpty(); 
    void displayQueue();
private:
    Node* back;
    Node* front;
};

Queue::Queue() {
    back = NULL;
    front = NULL;
}

Queue::~Queue() {
        while (!isEmpty()) {
        popFront();
    }
}

void Queue::pushBack(int d) {
    Node* temp;
    if (temp == NULL) {
    return;
    } else {
            temp->link = NULL; <========== This is where is get the error
            if (back == NULL) {
                  back = temp;
                  front = temp;
            } else {
                  front->link = temp; <===== here too
              front = temp;
            }
      }
}


bool Queue::popFront() {
    if (front == NULL) {
        return false;
    } else {
        Node* removeNode;
        removeNode = front;

        if (back == front) {
            back = NULL;
            front = NULL;
        } else {
            Node* previousFront = back;
            while (previousFront->link != front) {
                previousFront = previousFront->link;
            }

            front = previousFront;
            front->link = NULL;
        }

        delete removeNode;
        return true;
    }
}

bool Queue::isEmpty() {
    return (back == NULL);    
}

void Queue::displayQueue() {
    if (isEmpty()) {
        cout << "Queue is empty!" << endl;
    } else {
        Node *current;

        current = back;

        cout << endl << "-- BACK --  ";

        while (current != NULL) {
        cout << current->data << "  ";
        current = current->link;
        }

        cout << "-- FRONT --" << endl << endl;
    }
}

int main(){
    Queue q;
    q.displayQueue();
    q.pushBack(20);
    q.pushBack(30);
    q.displayQueue();
    q.pushBack(40);
    q.pushBack(12);
    q.displayQueue();
    q.popFront();
    q.displayQueue();

    return 0;
}