fix bug when saving diff pointing to initial diff

aircox/models.py

@@ -727,6 +727,7 @@ class Schedule(models.Model):
         if self.initial:
             self.program = self.initial.program
             self.duration = self.initial.duration
+            if not self.frequency:
                 self.frequency = self.initial.frequency
         super().save(*args, **kwargs)