import os


def find_largest_file(folder_path: str) -> str | None:
    """
    Find the largest file in a given folder.

    Only files directly inside ``folder_path`` are considered (non-recursive);
    subfolders are ignored. If multiple files share the maximum size, any one
    of them may be returned.

    Parameters
    ----------
    folder_path : str
        Path to the folder.

    Returns
    -------
    str or None
        The path of the largest file. None if no files are found or
        `folder_path` is invalid.

    Examples
    --------
    >>> find_largest_file('/path/to/folder')
    '/path/to/folder/largest_file.txt'
    """
    raise NotImplementedError("Complete this function.")