```python
def cell_at(grid: list[list[int]], row: int, column: int) -> int | None:
    """
    Returns the value at the specified row and column in the grid.
    Returns None if either row or column is out of bounds.
    """
    if not grid or not grid[0]:
        return None
    if row < 0 or row >= len(grid) or column < 0 or column >= len(grid[row]):
        return None
    return grid[row][column]

def diag_of(grid: list[list[int]]) -> list[int]:
    """
    Returns the values running down the grid's main diagonal from the top left.
    """
    if not grid or not grid[0]:
        return []
    return [grid[i][i] for i in range(len(grid))]
```