Exception has occurred: AttributeError 'NoneType' object has no attribute 'start'

[robml]

Wanted to raise this Exception I came across using just the suffix_tree.py file, and adding the following code block for testing:

if __name__=="__main__":
    tree = SuffixTree("abca$")
    tree.print_dfs()

Note the error also occurs even with just "abca".