I reproduced it before posting a reply earlier. The way round it is to start the sim, select one of the bots in the sim, then open mutations. (or so I thought)
Oh bugger, 400k cycles into a league match (DIN v The One) and managed to crash it by having another look.
I don't seem to be able to consistently avoid that problem; it can happen with a bot selected when you select robots/mutations or when you r/click on a bot and open mutations, doesn't seem to make much difference if sim is paused or running.
I presume you want to change mutation rates while you have a sim running, I know it is possible, some people do it. We'll just have to wait until they tell us how...
I only ever set mutations before start of sim so don't know how they do this, sry.
EDIT
In fact, I just thought I'd check on mutation rates before starting a new league match, it done the same thing, v nasty. May have to download latest version of DB again or restart pc just to make sure I haven't messed anything up.
EDIT
It seems I had to delete the bots from the 'loaded species' list and put them back again to stop the error occuring when I tried to open mutation rates before starting a new sim. Starting again with new bots and selecting a bot before opening robots/mutations seems to work again. Will see if I can find out what I did to make it go so weird.
Last EDIT
And now the only way I can get it to crash is by opening robots/mutations without a bot selected, V. Strange.