Update app.py

app.py

@@ -106,12 +106,19 @@ def follow(nt):
                     subrule = subrule[index_nt + 1:]
                     if len(subrule) != 0:
                         res = first(subrule)
-                        if '#' in res:
-                            res.remove('#')
-                            res += follow(curNT)
+                        if res is not None:
+                            if '#' in res:
+                                res.remove('#')
+                                follow_res = follow(curNT)
+                                if follow_res:
+                                    res += follow_res
+                        else:
+                            res = []
                     else:
                         if nt != curNT:
                             res = follow(curNT)
+                            if res is None:
+                                res = []
                     solset.update(res if type(res) is list else [res])
     return list(solset)