Java Programming

I want to make a screenshot and then convert the image to a byte of arrays so I can send it through a BufferedOutputStream.

try
		{
			robot = new Robot();
			screenshot = robot.createScreenCapture(new Rectangle(500,500));
			imageFile = new File("image.png");

			ImageInputStream iis = ImageIO.createImageInputStream(screenshot);

			byte[] data = new byte[1024];
			byte[] tmp = new byte[0];
			byte[] myArrayImage = new byte[0];
			int len = 0;
			int total = 0;
			while((len = iis.read(data)) != -1 ) // LINE 52 --- EXCEPTION CATCHED HERE
			{
				total += len;
				tmp = myArrayImage;
				myArrayImage = new byte[total];
				System.arraycopy(tmp,0,myArrayImage,0,tmp.length);
				System.arraycopy(data,0,myArrayImage,tmp.length,len);
			}
			ios.close();

I get this exception while running:

Exception in thread "Thread-0" java.lang.NullPointerException
at Server.run(Server.java:52)
at java.lang.Thread.run(Unknown Source)