Leetcode problem - Vertical Order Traversal of a Binary Tree

Problem Link - https://leetcode.com/problems/vertical-order-traversal-of-a-binary-tree/

I am getting a following error on leetcode :-

Line 1034: Char 9: runtime error: reference binding to null pointer of type ‘std::vector<int, std::allocator>’ (stl_vector.h) SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/…/lib/gcc/x86_64-linux-gnu/9/…/…/…/…/include/c++/9/bits/stl_vector.h:1043:9

Kindly Suggest a possible correction to my code

My Code :-

      /**
      * Definition for a binary tree node.
      * struct TreeNode {
      *     int val;
      *     TreeNode *left;
      *     TreeNode *right;
      *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
      *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
      *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), 
          right(right) {}
     * };
   */
    class Solution {
   public:
   vector<vector<int>> verticalTraversal(TreeNode* root) 
  {

       vector<vector<int>> ans;

       map<int, vector<int>> mp;

       if(root == NULL)
           return ans;

       queue<pair<TreeNode* , int>> q;
       q.push({root,0});

 
       while(!q.empty())
       {
           auto p= q.front();
           TreeNode *curr = p.first;
           int hd=  p.second;
           q.pop();
    
    
          if(root->left != NULL)
         {
             q.push({root->left, hd-1});
         }
    
        if(root -> right != NULL)
        {
            q.push({root->right, hd+1});
        }
        ans[hd].push_back(curr->val);
    }

   return ans;
 }
 };

Try initialising the vector or handle the case where index hd is not there in ans.

if (hd >= ans.size())
{
        vector<int> temp;
        temp.push_back(curr->val);
        //if hd == y and vector.size == x, insert an empty vector at indices (x,y]
        ans.push_back(temp);
}
else
{
        ans[hd].push_back(curr->val);
}