Hashtable().put is throwing NPE

So I'm trying to make a simple game and I want the images loaded from the start so I want to create a hashtable that stores the Image with its name as the key. When I call ImageStore.get().getImage("image name") from my gameloop, I get an error when the ImageStore is initialized (shown as a comment in the code).

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public class ImageStore {

   private static ImageStore single = new ImageStore();

   public static Hashtable<String, Image> images = new Hashtable<String, Image>();

   private ImageStore() {

      BufferedImage sourceImage = null;

      String dirPath = "images";
      File imageDir = new File(dirPath);
      imageDir = new File(imageDir.toURI());
      String absoluteDirPath = imageDir.getPath();
      absoluteDirPath = absoluteDirPath.replace('\\', '/');
      imageDir = new File(absoluteDirPath);
      File[] imageFiles = imageDir.listFiles();

      if (imageFiles.length == 0)
         System.out.println("Directory empty");
      else {
         for (int i = 0; i < imageFiles.length; i++) {
            String ref = imageFiles[i].getName();
         
            try {
               sourceImage = ImageIO.read(imageFiles[i]);
            } catch (IOException e) {
               fail("Failed to load: " + ref);
            }
            
            GraphicsConfiguration gc = GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice().getDefaultConfiguration();
            Image image = gc.createCompatibleImage(sourceImage.getWidth(), sourceImage.getHeight(), Transparency.BITMASK);
            
            //both of these print values without problems, so neither argument
            //passed to the hashtable is null
            System.out.println(image.getWidth(null));
            System.out.println(ref);

            //getting a null pointer exception here
            images.put(ref, image);
         }
      }
   }

Also if I'm doing anything in a weird way in the code, let me know of the proper way (I'm new to game programming). Thanks in advance for any help.