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

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".