Explicitly set Timer.duration to None on instance save when the instance is still running.

This has to be done because the Timer.from_db() method fills in duration against the current time when the instance is still active. This issue was introduced in fd960cd.

core/models.py

@@ -183,6 +183,8 @@ class Timer(models.Model):
         self.name = self.name or None
         if self.start and self.end:
             self.duration = self.end - self.start
+        else:
+            self.duration = None
         super(Timer, self).save(*args, **kwargs)