Steps to produce: 
1. Run Main.java
2. When run, the program abruptly stops and throws out the NullPointerException.

Actual Results: Exception occurred and program stopped
Expected Results: Need to show the exception that occurred


//Case when the Arraylist of slots is null
@Test
public void testOptimize13() {
	
	ByteArrayOutputStream outContent = new ByteArrayOutputStream();
    System.setOut(new PrintStream(outContent));
	
    Item item1 = new Item(1,new Day("13-Oct-2020"),new Day("20-Oct-2020"), 1);
	Item item2 = new Item(5,new Day("13-Oct-2020"),new Day("20-Nov-2020"), 2);
	Item item3 = new Item(4,new Day("13-Oct-2020"),new Day("25-Oct-2020"), 3);
	Item item4 = new Item(1,new Day("13-Oct-2020"),new Day("25-Oct-2020"), 4);
	
	ArrayList<Slot> slots = null; //new ArrayList<>();
        Warehouse.createInstance(slots);
	Warehouse.getInstance().printItemsInSlots();
	
	//denny addition
	ArrayList<Item> items = new ArrayList<>();
	items.add(item1);
	items.add(item2);
	items.add(item3);
	
	//denny addition
	Warehouse.getInstance().optimize(items);
    
	assertEquals("NullPointerException Occured", outContent.toString().trim().substring(outContent.toString().trim().lastIndexOf('\n')+1));
	
}