Undos need to persist after previewing
I'm really new to GS and during my exploration I've been changing and deleting behaviours to see what effect they had only to go back to the behaviour panel and lost my option to undo my last change. Not the greatest way to learn 
~t
~t
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Comments
This allows you to switch behaviour's on and off - meaning you don't need to delete them and can do exactly what you're trying!
Hope that helps!
QS
I always do a save as to save a new version before I delete or change something significant so I always have a way back.
Cheers,
Ludwig
~t