Iterate over collection of transactions using iterator

The method below is basic insight of what I am planning to output: the account part works but when I try to print along with the transactions I get the nullpointer exception.

The reason is I am using my own queue implementation, underneath the method is my iterator so far but I have no clue what I am missing for this to not work?

         public void Display_Account() {
         System.out.println("Student ID: " + Student_ID);
         System.out.println("Student Name: " + Student_Name);
         System.out.println("Student Email: " + Student_Email);
         System.out.println("Creation Date (DD/MM/YYYY): " + Creation_Date);
         System.out.println("Credit: " + Credit_Balance);
         System.out.println("*----------------------*");

         // Code indicates not applicable to expression type and requires iterable 
         to be implemented
         for (Student_Transaction Tr : Transaction) { 
             Tr.Display_Transaction();
         }
      }

Therefore, I will need to implement the Iterator in my queue class which is the data structure I am using to add or remove objects, despite I'm missing few errors
but I dont know where. 

      // Queue uses class List.
      import java.util.Iterator;

      public class Queue <t> implements Iterable <t>
      {
       private List<t> queueList;
       private int capacity; 

       // no-argument constructor
       public Queue() 
      { 
         queueList = new List<>("queue"); 
         capacity = 1;
      } // end Queue no-argument constructor

      @Override
      public Iterator<t> iterator() {
      return null;
      }

      class QueueIt implements Iterator<t> { 

        // Where the iterator problem is in particular!
   public class QueueIt implements Iterator<t> { 

        @Override
        public boolean hasNext() {
            return false;
        }

        @Override
        public T next() {
            return null;
        }
   }

    
   // add object to queue
   public void enqueue(T object)
   { 
      queueList.insertAtBack(object); 
      capacity++;
   } // end method enqueue

   // remove object from queue
   public T dequeue() throws EmptyListException
   { 
      return queueList.removeFromFront(); 
   } // end method dequeue

   public void Decrement() 
   { 
       capacity--;
   }
   public int Size() 
   { 
       return capacity; 
   }
   
   // determine if queue is empty
   public boolean isEmpty()
   {
      return queueList.isEmpty();
   } // end method isEmpty

   // output queue contents
   public void print()
   {
      queueList.print();
   } // end method print
}

What I have tried:

Every example I have found are related to LinkedList. Dont get me wrong Queue is a member of LinkedList and the bigger picture (Collections utility and iterator). But I am not sure what I am missing out that my Code wont Iterate through objects. 

P.S If you need to see my List class just say so and I will edit the Snippet above.