java.lang.NullPointerException: Cannot invoke "java.util.List.size()" because "<local4>.children" is null
  at line 134, Serializer.serializeRoot
  at line 188, __Driver__.main

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val, List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

class Codec {
    // Encodes an n-ary tree to a binary tree.
    public TreeNode encode(Node root) {
        if(root == null) return null;
        
        TreeNode r = new TreeNode(root.val);
        if(root.children != null 
           && !root.children.isEmpty()){
            TreeNode child = encode(root.children.get(0));
            r.left = child;
            for(int i=1;i < root.children.size();i++){
                child.right = encode(root.children.get(i));
                child = child.right;
            }
        }
        return r;
    }
    
    // Decodes your binary tree to an n-ary tree.
    public Node decode(TreeNode root) {
        if(root == null) return null;
        Node ans = new Node(root.val);
        if(root.left != null){
            ans.children = new LinkedList<>();
            TreeNode child = root.left;
            while(child != null) {
                ans.children.add(decode(child));
                child = child.right;
            }
            
        }
        return ans;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.decode(codec.encode(root));