That is because no ElectrostaticDifferencePotentials analysis are present in your lih2li_iv_analysis.nc file.
See line 36/37 in lih2li_iv_analysis.py which is commented out.
The voltage drops are already stored in your lih2li_iv_analysis.nc file. See the lines:
if float(bias)!=0.:
voltage_drop = potential - zero_bias_potential
nlsave(analysis_filename, voltage_drop, object_id="Voltage drop %s" % bias)
You can analyze them directly with the 1D projector plugin instead of using Custom Analyzer (kind of outdated tool)