C++ program stops execution without errors when running a class setter

I am writing a Sudoky program to practice C++. I have a Sudoku class with a populateGrid method that populates the Sudoku grid with appropriate values. The sudoku grid is a 2d array of Cells (Cell being another class I made). The Cell class has a method called setNumber, to set the number of that particular cell.

Here is the populateGrid method in the Sudoku class:

void Sudoku::populateGrid()
{
    unsigned int i, j;

    for (i = 0; i < this->rows; i++)
    {
        for (j = 0; j < this->cols; j++)
        {
            std::vector<int> validValues = this->findValidValues(i, j);

            int proposedValue = validValues.at(rand() % validValues.size());
            this->grid[i][j].setNumber(proposedValue);
        }
    }
}

Note that the setNumber method is being called at the end of the inner for loop.

This is the code of setNumber in the Cell class:

void Cell::setNumber(int n)
{
    if (n <= 0 || n > 9)
    {
        std::cout << "Error: Invalid Argument" << std::endl;
    }
    this->number = n;
}

The program simply stops execution when tryng to run this->number = n; in the setNumber method. It doesn't show any errors during compiling, nor runtime. If I sorround that line with a try-catch block, I am not catching any exceptions.

I honestly don't know what the issue might be. I would be very thankful if anyone had an idea. Please let me know if you need more details on the program.

Thanks in advance!

EDIT:

this is how the grid is being initialised:

Sudoku::Sudoku(int rows, int cols)
{
    this->setRows(rows);
    this->setCols(cols);

    this->grid = new Cell *[this->rows];

    unsigned int i, j;

    for (i = 0; i < this->rows; i++)
    {
        this->grid[i] = new Cell[this->cols];

        for (j = 0; j < this->cols; j++)
        {
            this->grid[i][j] = Cell();
        }
    }

    this->populateGrid();
    this->hideCells();
}

The type of the grid is Cell**. As mentioned previously, the program stops running when executing this->populateGrid().