Card doesn't show, TypeError: Cannot read properties of undefined (reading 'length')

[bvhme]

On pages where I put a card, without showing it anything, I get

TypeError: Cannot read properties of undefined (reading 'length')

The code at that address is:

if( chartData.length > 0 ) 

This is my config and those sensors are the right ones:

type: custom:neerslag-card
title: Neerslag
entities:
  - sensor.buiten_buienalarm_regen_data
  - sensor.buiten_buienradar_regen_data

[aex351]

Error management of the card could be improved to cache such errors. However, it is slightly an edge case as we can expect any of the used sensors to contain data.