How to prevent "what(): basic_string::_M_construct null not valid" error?

I am building a tic tac toe game just for the sake of my own practice. I encountered this error, when trying to run the file. terminate called after throwing an instance of 'std::logic_error' what(): basic_string::_M_construct null not valid Aborted (core dumped) I dont know where and why exactly I am getting this error. Any leads would be highly appreciated. This is my code.

#include <iostream>
#include <stdlib.h>
#include <cstdlib>
#include <string>
using namespace std;


string defaultBoard = "__";
string defaultPlayer1 = "X";
string defaultPlayer2 = "Y";

void createBoard(string **board, int size){
    for (int i=0; i < size; i++){
        for (int j=0; j < size; j++){
            board[i][j] = defaultBoard;
        }
    }
}

void displayBoard(string **board, int size){
    for (int i=0; i < size; i++){
        for (int j=0; j<size; j++){
            cout<<board[i][j]<<"  ";
        }
        cout<<endl<<endl;
    }
}

int *playerTurns(int playerTurn, int size, string **board){
    cout<<"PLayer " + to_string(playerTurn) + " turn"<<endl<<endl;
    displayBoard(board, size);
    static int coords[2];
    while(true){
        cout<<"Enter x and y of the turn seperated by space eg. (1 2):";
        int x;
        int y;
        cin>>x>>y;
        if (x >= size  || y >= size){
            cout<<"Invalid credentials"<<endl;
        } else if (board[x][y] != defaultBoard){
            cout<<"Place already filled"<<endl;
        } else{
            if (playerTurn == 1){
                board[x][y] = defaultPlayer1;
            } else{
                board[x][y] = defaultPlayer2;
            }
            coords[0] = x;
            coords[1] = y;
            system("clear");
            break;
        }
         
    }
    return coords;
}

bool playerWin(string **board, int size, int player_id, int x, int y){
    string defaultPlayers[2] = {defaultPlayer1, defaultPlayer2};
    bool x_row = true;
    bool y_row = true;
    bool diag_row = true;
    for (int i=0; i<size; i++){
        if (board[x][i] != defaultPlayers[player_id-1]){
            x_row = false;
        }
        if (board[i][y] != defaultPlayers[player_id-1]){
            y_row = false;
        }
        if (x == y){
            if (board[i][i] != defaultPlayers[player_id-1]){
                diag_row = false;
            }   
        }
    }
    if (x_row || y_row || (diag_row && x==y)){
        return true;
    } else{
        return false;
    }
}

int main(int argc, char **argv){
    int size = 0;
    string sizeGiven = argv[1];

    try{
        size = stoi(sizeGiven);

    } catch(...) {
        cout<<"Invalid size"<<endl;
        exit(1);
    }
    
    if (size <=0){
        cout<<"Invalid board size"<<endl;
        exit(1);
    }

    string **board = new string*[size];
    for (int i = 0; i < size; i++){
        board[i] = new string[size];
    }

    createBoard(board, size);

    int *coords;
    while(true){
        coords = playerTurns(1, size, board);
        cin.ignore();
        cin.clear();        
        // displayBoard(board, size);
        if (playerWin(board, size, 1, coords[0], coords[1])){
            cout<<"Player 1 has won"<<endl;
            break;
        }
        coords = playerTurns(2, size, board);
        cin.ignore();
        cin.clear();
        if (playerWin(board, size, 2, coords[0], coords[1])){
            cout<<"Player 2 has won"<<endl;
            break;
        }
        // displayBoard(board, size);
    }


    return 0;
}