This is DSA Question. I’m getting at error at runtime

error: reference binding to null pointer of type ‘int’ (stl_vector.h)

I’m getting this error in leetcode while submitting, but it is working fine in my local system.

 #include<bits/stdc++.h>
    using namespace std;
    #define ll long long
    #define nl 'n'
    #define py cout << "YES" << endl
    #define pn cout << "NO" << endl
    
    int main(){
        
        vector<int> nums = {0,2,3,4,6,8,9};
        int n = nums.size();
    
        string str = to_string(nums[0]);
        
        vector<string> ans; 
    
        for(int i=1; i<n; i++){
            char lastChar = str[str.length()-1];
            int no = lastChar - '0';
    
         
            if(nums[i] == no+1){
                if(str.length() > 1){
                    str[str.length() - 1] = '0' + nums[i];
                }else{
                    str += "->" + to_string(nums[i]);
                }
            }else{
                ans.push_back(str);
                str = to_string(nums[i]);
            }
        }
    
        ans.push_back(str);
        
    
        for (auto &&i : ans)
        {
            cout << i << " ";
        }
        
        return 0;
    }