self.mixer has no attribute getmute once time passes

[RalphCorderoy]

volti 0.2.3-5 on Arch Linux. volti starts fine and works: Button2 toggles mute, slider adjusts volume. Leave overnight, including switching VTs to other X users running volti. volti still present, but slider now has no effect as it adjusts, and Button2 produces

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/volti/menu.py", line 56, in on_toggle_mute
    self.main.toggle_mute(widget)
  File "/usr/lib/python2.7/site-packages/volti/main.py", line 346, in toggle_mute
    volume = self.get_volume()
  File "/usr/lib/python2.7/site-packages/volti/main.py", line 293, in get_volume
    if self.alsactrl.is_muted():
  File "/usr/lib/python2.7/site-packages/volti/alsactrl.py", line 153, in is_muted
    if self.mixer.getmute()[0] == 1:
AttributeError: 'NoneType' object has no attribute 'getmute'

Something has caused self.mixer to become None. A cursory look suggests that can only happen with close().