How can I only refer to an object ONLY if it actually exists?

I am implementing a stack using linked list in java. The problem is that I get a nullPointerException when there is no element below, e.g. the StackNode.link does not exist. Therefore if I try assigning the StackNode.link I get the Exception.

Using an if statement to only run the code if it exists, I just get the Exception in the if statement. How do I go about this?

int pop() {

    StackNode temp = top;

    // update top
    top = belowTop;
    belowTop = top.link; // this is where I get the nullPointExcpetion


    return temp.data;

}

I would expect that when top.link does not exist (e.g. is null) then belowTop would just be null. This would be fine, but as described I get the exception.

EDIT: This is what I tried with the if-statement

if (top.link != null) {
        belowTop = top.link;
    }