TypeError: 'NoneType' object is not iterable

[Elenikary]

I ve been trying to run the test_wfse.py file and I have been getting a lot of errors which are referencing the wfse.py and wfse_product.py files. I was able to fix the ones referencing the wfse.py file and now I have to been facing the ones referring to the product function. This one error I am getting now refers to line 49 from wfse_product.py:

                                                   **for wfse_out in wfse.next_state(init_wfse, init_prop):**

As I was investigating the error, I looked up where the function next_state() is defined and I found it in the fsa.py file. This function returns None for the given inputs and therefore it cannot iterate with the for loop (?). Also I noticed from the comments in file fsa.py that None is returned in the cases of "blocking automata". Does that mean that the wfse created as is now is returning a blocking automaton?
This is the error that I am getting from running the test_wfse.py file:

Traceback (most recent call last):
File "/Users/eleni/Documents/lomap-features-p23/lomap/tests/test_wfse.py", line 93, in
product_model = product_function(ts, wfse, fsa)
File "/usr/local/lib/python3.7/site-packages/lomap/algorithms/wfse_product.py", line 49, in product_function
for wfse_out in wfse.next_state(init_wfse, init_prop):
TypeError: 'NoneType' object is not iterable