Fix Nonetype exception

sonogram.py

@@ -205,7 +205,8 @@ class Sonogram():
             if decision == 'group':    
                 memberSet = set()
                 for j in range(i,end):
-                    memberSet.add(sas[i])
+                    if sas[i] is not None:
+                        memberSet.add(sas[i])
                 # Convert to list and sort for convenience
                 memberSet = list(memberSet)
                 memberSet.sort()