I talked to an Adobe support person on the phone a month or so back and they said Adobe's OpenCL and Mac High Sierra aren't playing well together. They told us to switch to the Metal renderer via project settings. Its a bit slower than the OpenCL but we haven't had the same problem with crashing.
You also have to delete your old preview files and a few other things on project's that were already using the OpenCL but I don't remember what they were off the top of my head. I think it was something with pointing premiere to a new 12.0 folder.