"""Parse a string of topics into a list of unique topics.

This function takes a string of topics separated by commas, splits it into individual topics, 
strips any leading or trailing whitespace, and removes any duplicate topics while preserving 
the original order.

Args:
    topics_string (str): A string of topics separated by commas. If the string is empty or 
                         contains only whitespace, an empty list is returned.

Returns:
    List[str]: A list of unique topics in the order they first appeared in the input string.

Examples:
    >>> parse_topics_string('topic1')
    ['topic1']
    >>> parse_topics_string('topic1,topic2,topic3')
    ['topic1', 'topic2', 'topic3']
    >>> parse_topics_string('topic1, topic2, topic3')
    ['topic1', 'topic2', 'topic3']
"""
You should write code starting with:
from typing import List

def parse_topics_string(topics_string: str) -> List[str]:
    # TODO: Implement this function based on the docstring.
    pass

