Initializing a smart pointer to an instance of class and accessing its methods

So, I'm trying to implement a priority queue using a sorted list. I have my priority queue class inherit from my sorted list class. My sorted list class works fine when I test its functions in the driver, but I am getting "Segmentation Fault 11" when I try testing out my implementation of the priority queue. I ran my code through the CLion debugger and got the error "EXC_BAD_ACCESS (code=1, address=0x0)", which, after some research, seems to be caused by trying to modify data in a null pointer. This is my first time using smart pointers, so I think my issues lie in a misunderstanding with how they're constructed and initialized.

Here's my header file for my SL_PriorityQueue class:

#ifndef PRIORITY_QUEUE_
#define PRIORITY_QUEUE_

#include "PriorityQueueInterface.h"
#include "LinkedSortedList.h"
#include "PrecondViolatedExcep.h"
#include <memory>

template<class ItemType>
class SL_PriorityQueue : public PriorityQueueInterface<ItemType>
{
  private:
     std::unique_ptr<LinkedSortedList<ItemType> > slistPtr;

  public:
    SL_PriorityQueue();
    SL_PriorityQueue(const SL_PriorityQueue& pq);
    ~SL_PriorityQueue();

    bool isEmpty() const;
    bool enqueue(const ItemType& newEntry);
    bool dequeue();

    //@throw PrecondViolatedExcept if priority queue is isEmpty
    ItemType peekFront() const throw(PrecondViolatedExcep);
};

#endif

Here's the driver I'm using to test my code:

#include "../src/Node.cpp"
#include "../src/LinkedSortedList.cpp"
#include "../src/SL_PriorityQueue.cpp"
#include <iostream>

int main()
{
    std::shared_ptr<SL_PriorityQueue<int> > testing (new SL_PriorityQueue<int>());
    testing->enqueue(7);
    std::cout << testing->peekFront() << std::endl; //I set a break point here, which is where CLion throws the exception
    std::cout << testing->dequeue() << std::endl;
    std::cout << testing->isEmpty() << std::endl;
    return 0;
}

Here's the function from SL_PriorityQueue.cpp that the CLion highlighted after the exception:

template <class ItemType>
bool SL_PriorityQueue<ItemType>::enqueue(const ItemType& newEntry)
{
  slistPtr->insertSorted(newEntry);
  return true;
}

The error makes me think that when I call insertSorted in the above function, slistPtr is still null. My constructor for SL_PriorityQueue is empty since it seems that with smart pointers they manage their own memory such that I don't have to set it equal to a null pointer.

I tried making slistPtr a shared pointer to see if maybe too many things were pointing to it, but I just received the same error.

Thanks in advance for any help you can provide!