This may be a bug in Synth1 but even so, it shouldn’t crash Renoise right?
Yes, that’s very likely a bug in Synth1. Should not, yes, but unfortunately Renoise can’t catch all errors that plugins may cause, especially not when they are happeing in the plugins UI (event handlers).
Try 32 bit version of synth1. The 64bit build is buggy, at least on macos. If you are on macos, you can strip off the 64bit binary from the universal binary, so even Renoise 64bit will load the 32bit bit synth1 thru the bridge.