There is a rare case where you need to fix the splatmap setup. It can happen if there is an error during splatmap processing. If you get a fatal error complaining about an index being out of bounds, check if the 2nd splatmap field is really empty ("none") or if there is a blank (all black) splatmap there. This black splatmap is generated internally during processing and usually cleaned up at the end. But sometimes, it gets stuck. Simply click on "select" and set the 2nd field to "none" and things should be all fine.