Better organize the code.

Not sure if this is overkill, but it sure is cleaner. :)

Actually, the getAgreeNode function already checks for nid:

public function getAgreeNode() {  
  if (!$this->getNid()) {  
    return NULL; 
  }  
  return $this->entityTypeManager->getStorage('node')->load($this->getNid());

So using both would be redundant.

But the getAgreeNode function needs an else statement.

Actually, I like this better than the else statement:

  /**
   * {@inheritdoc}
   */
  public function getAgreeNode() {
    $node = NULL;
    if (!empty($this->getNid())) {
      $node = $this->entityTypeManager->getStorage('node')->load($this->getNid());
    }
    return $node;
  }