It seems like the generator thinks that there is a room template without any tiles. Unfortunately, the exception doesn’t contain the room template name, which I should fix in the future.
However, when you open a room template prefab, you should see a status panel in the top-left corner saying whether the room template is valid or not (if you don’t see the status panel, make sure there is the Room Template Settings component attached and that you allow gizmos from that component). If the room template is not valid, there should also be a red warning in the Room Template Settings component. So you should be able to find the wrong room template quite easily.
Let me know if you need any additional help!