Local FFT buffer – that’s odd – did you observe this in a debugger trace?
I’ve had hiccups before where I freed a buffer, but the synth(s) using it hadn’t been fully removed yet. In your onClose function, you’re freeing the synth first but, if NodeProxy releases the synth rather than brute-force freeing it, then the synth may still be in its release phase when you free the buffers. If there’s a UGen in the synth that accesses the buffer and doesn’t guard against the buffer disappearing, that could cause a crash.
hjh