OK, So that helps narrow it down…
based on the exception it looks like the issue is on line 735 which is this:
if (this.config.entity_pop && !this.config.alt_pop) { root.getElementById("pop-text").textContent = `${Math.round(this._hass.states[this.config.entity_pop].state)}` }
So that line is looking to find the “pop” current value somewhere in the card. Its doing that because you have entity_pop defined in your configuration.
The issue I see based on your screen shot of the card is that you are not showing “pop”. I am guessing you are using the slot configuration flags for all eight slots with some of them set to “empty” or “remove” and none of them set to “pop”.
Therefore you are telling the card you want to use pop but then not showing it. Remove the entity_pop: line from your configuration and see if that helps. Based on your card screen shot you should also make sure you are not defining any of the other optional entities that you are not showing. (entity_sun and entity_daytime_high) as they will have the same issue.
In a future version of the card I will add some logic to prevent it from trying to update items that are intentionally hidden while still being configured.