`solaar-cli show -v` crashes when a Performance MX is charging

Created by: WhyNotHugo

When my device is charging, solaar-cli show -v crashes:

$ solaar-cli show -v
Unifying Receiver
   Device path  : /dev/hidraw0
   USB id       : 046d:c52b
   Serial       : 225C9B9A
     Firmware   : 12.01.B0019
     Bootloader : 02.14
   Has 1 paired device(s) out of a maximum of 6 .
   Notifications: 0x000900 = wireless, software present
   Device activity counters: 1=78

1: Performance Mouse MX
   Codename     : Performance MX
   Kind         : mouse
   Wireless PID : 101A
   Protocol     : HID++ 1.0
   Polling rate : 8 ms
   Serial number: 7B0331A0
        Firmware: 15.01.B0062
      Bootloader: 02.11
           Other: 00.09
   The power switch is located on the base.
   Notifications: (none).
Traceback (most recent call last):
  File "/usr/bin/solaar-cli", line 42, in <module>
    solaar.cli.main()
  File "/usr/lib/python3.5/site-packages/solaar/cli.py", line 431, in main
    args.cmd(receiver, args)
  File "/usr/lib/python3.5/site-packages/solaar/cli.py", line 207, in show_devices
    _print_device(dev, args.verbose)
  File "/usr/lib/python3.5/site-packages/solaar/cli.py", line 190, in _print_device
    text = '%d%%' % level
TypeError: %d format: a number is required, not NoneType