I was asked to check the object returned by dimensionChange() but In Main class I am getting null pointer exception error. How can I fix this? - nullpointerexception

Question

The method dimensionChange() in Rectangle class returning null is the given condition and it is asked to check whether the object returned by dimensionChange() is an instance of class using instanceof operator.
I can understand that the object returned by dimensionChange() is null but how can i check the returned object with instanceof operator without getting nullpointerexception error.
Any solution could be greatly helpful.
public class Main {
public static void main(String[] args) throws Exception, IOException {
//Fill your code
int l,b,n;
boolean h;
Scanner sc=new Scanner(System.in);
System.out.println("Enter the length of the rectangle");
l=sc.nextInt();
System.out.println("Enter the width of the rectangle");
b=sc.nextInt();
Rectangle r=new Rectangle(l,b);
r.display();
System.out.println("Area of the Rectangle:"+r.area());
System.out.println("Enter the new dimension");
n=sc.nextInt();
Rectangle r2=r.dimensionChange(n);
r.display();
System.out.println("Area of the Rectangle:"+r.area());
h=r2 instanceof Rectangle; **//null pointer exception**
}
}
public class Rectangle {
//Fill your code
private int length,width;
public Rectangle(int length,int width)
{
this.length=length;
this.width=width;
}
public Integer area(){
//Fill your code
return length*width;
}
public void display(){
//Fill your code
System.out.println("Rectangle Dimension");
System.out.println("Length:"+length);
System.out.println("Width:"+width);
}
Rectangle dimensionChange(Integer newDimension){
Rectangle rectangleObject = null;
//Fill your code
length=newDimension*length;
width=newDimension*width;
return rectangleObject;
}
}