Zone cell and thermostat panel don't recognize a stale sensor

[climategadgets]

• Expected behavior: both the zone cell and the thermostat panel indicate when the sensor for the disabled zone is stale (missing).
• Actual behavior:
  • The only indication is the gap on the temperature chart, and setpoint display (see #36);
  • The current temperature reading, instead of being --.-, is stuck at the last known value.