Setup - xLights BETA 2017.33 64 Mac (I also tested on the same 64bit version in Windows and had the same result)
I recorded this to better understand the issue. Here is the video link:
https://drive.google.com/open?id=0B7E6agkvZj6wWW9Vd1p1alhpSUkI'll walk you through the video:
1 - I show you that I've created the model group "Whole House No LM,GA,HF1" and what is inside of it
2 - I then add the model group "Whole House No LM,GA,HF1" to the model group "Whole House" which appears to work until you click away and then back to find it added another model group in its place "Whole House No LM" which is obviously not what I selected
3 - I show step 2 once more to prove I'm not crazy and clicked the wrong thing

4 - Since it was adding / changing what I selected from "Whole House No LM,GA,HF1" to "Whole House No LM" I thought I'd try to add "Whole House No LM,GA,HF1" to the model group that it was changing it to "Whole House No LM" to see what happens. As you can see, it immediately crashes xLights
So I believe I've found a bug. I'm in the middle of cleaning up some of my model groups and consolidating things for ease of management but this is a blocker, at least for that. Hopefully it's an easy fix. I've also attached the log file package. Thanks for the help!