java 8 lambda expresssions exception handling

package syout;
import java.util.ArrayList;

public class Lambda {
        
   ArrayList<Node> al;

   // inner class
	private class Node{ 
		double[] a;
		
	}
	
	public Lambda(){
		al = new ArrayList<Node>();
		al.add(new Node());
		al.add(new Node());
		al.add(new Node());
		al.add(new Node());
	}
	
	
	public static void main(String[] args) {
		
		Lambda l = new Lambda();
		try{				
			l.al.forEach( n ->
				System.out.println(n.a[2]*n.a[3])
				);
		} catch (java.lang.NullPointerException e) {
			System.out.println(e);
		}
		
		
	}

}

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when i ran above code i got below exception.
please advise
java.lang.NullPointerException