"""
list_to_string(n):
- Input: list1, a list of strings .
- Output: a string that is the concatenation of all strings in list1.
- Error handling: returns None if list1 is not a list, or if
  any element of the list is not a string

A character is defined to be a string of length 1.
"""
def list_to_string(list1):
    if not(type(list1) is list):
        return None

    result = ""
    for i in list1:
        if not(type(i) is str):
            return None

        result = result + i;

    return result