Hi there!
The problem is happening here:
There is a method that gets the generator you selected to apply to the scene. To properly get the generator, the plugin needs you to tell the generator index you want.
In the image below you can see the numbers on the left side of each generator you created. The index value of each generator is that number - 1. So, the first generator is index 0. The second generator is index 1. And so on.
You can also use the generator id, so the plugin will automatically find the index that belongs to this ID.
The problem is, that the plugin is not finding that generator. My guess is that there may be something wrong between your plugin command and your plugin parameter. So, try double-checking if you inserted the right ID or INDEX on the plugin command(Also make sure the BUILD GENERATOR FILE parameter is set to true, just in case.).
If still, the problem is still not solved, send me a detailed screenshot of your plugin parameter > Generator List and the plugin command you are using on the event.