Assertion Error: Invalid Null Pointer

[Dan3444]

Hello everyone, I am learning c++ migrating from Visual basic. I have created a sample program in which a random number is generated and a user must guess what the number is. Unfortunately I am being bogged down by an assertion error, and I cannot figure out what is wrong with my code. Here is my code, and the error occurs at line 33. Any help would be much appreciated!

#include "stdlib.h"
#include "stdio.h"
#include "string.h"
#include <sstream>
#include <iostream>
#include <vector>

using namespace std;

int main()
{
char thing = NULL;

vector<int> randNum(4);
vector<int> guesses(4);

cout << "Enter any number \n";

int awesome;
cin >>  awesome;
srand(awesome);

cout << "Guess the four correct digits \n";

int bulls = 1;
int cows = 1;
string input = 0;

while (bulls < guesses.size()) {

	bulls = 0;
	cows = 0;
	
	cout << "Enter Four Digits";
		cin >> input;

		
		
		

for(int i = 0; i< randNum.size(); i++) {
		randNum[i] = rand() &#37; 10;
		guesses[i] = atoi(input.substr(0, 1).c_str());
		//5 = 4359

	}


	
for (int ii = 0; ii < randNum.size(); ii++) {
	for(int j = 0; j < guesses.size(); j++) {
		if(guesses[j] == randNum[ii] && j == ii){
			bulls += 1;	
			}
		else if(guesses[j] == randNum[ii]){
		 cows += 1;
			}
		} //end for (int j)
	} //end for(int ii)
if (bulls < guesses.size()) {
	cout << " You have guessed" << bulls << " bulls and " << cows << " cows";
	}
else
	{
		"Congratulations! The number you have guessed is correct!";
	} //end  if(bulls < 4)
	} // end while bulls < 4



	cin >> thing;
    //keeps the console window open

return 0;

	}