Getting error while using 2-d array through vectors

Question

I was coding on leetcode and while doing so I got this error - "Line 1034: Char 9: runtime error: reference binding to null pointer of type 'int' (stl_vector.h)" Can anyone please help me where I am doing wrong. Below is my code

class Solution {
public:
int uniquePaths(int m, int n) {
    vector<vector<int>> ans;
    vector<int> help;
    for(int i=0;i<m;i++)
    {
        for(int j=0;j<n;j++)
        {
            vector<int> temp;
            if(i==0||j==0)
            {
                temp.push_back(1);
            }
            else
            {
                temp.push_back(temp[j-1]+help[j]);
            }
            help=temp;
            ans.push_back(temp);
        }
    }
    return ans[m-1][n-1];
}