I believe I've found a bug during track freeze: Whenever I select many tracks to freeze, REAPER consistently crashes after render, when "locking" items and removing FX. This happens in 5.12/64bit and also 5.15/64bit pre-release on Mac OS X. It does *not* happen on 5.12/32bit Mac OS X...
It's probably related to a particular plug-in which is being offlined after the freeze, you might want to try different plug-ins and see if it is a particular plug-in.
Here's the Crash Report, attached. I thought It was a plugin issue too.. I'm using FabfFilter, SoundToys and UAD plugins, but the same session didn't crash when opened in REAPER 32bit... ?