Avoid crashing when a fit is not found.

naive-nlu/tree_nlu/knowledge_base.py

@@ -53,13 +53,16 @@ class KnowledgeBase(object):
 
         return knowledge_diff_getter
 
-
     def process(self, row):
         row = row.lower()
         knowledge_before = copy.deepcopy(self.knowledge)
         logging.debug("\x1b[7;32m> {} \x1b[0m".format(row))
         tokens = parsing.to_tokens(row)
-        tokens, inferred_tree = parsing.get_fit(self, tokens)
+        fit = parsing.get_fit(self, tokens)
+        if fit is None:
+            return None
+
+        tokens, inferred_tree = fit
         result = knowledge_evaluation.integrate_information(self.knowledge,
                                                           {
                                                               "elements": tokens,