from typing import Dict
from typing import Optional


def dict_value_to_str(obj: Dict, key: str) -> Optional[str]:
    """
    Convert the value referenced by the key in the dict to a string, if it exists, and return it. If it doesn't exist,
    return None.
    """
    value = obj.get(key)
    if value is not None:
        return str(value)
    else:
        return None