The crash is due to a corrupted file.
Look for ../config/island.json and remove that file - it will be created on next start up.
Also - you're currently allocating all available memory you have on your machine, that is not a good idea - you need to leave some for the OS and other applications. Worst scenario the game bugs out, hogs all memory and your computer crashes - and if you're unlucky, you'll end up with a corrupted world (and perhaps other issues on your machine not related to minecraft).
Reduce allocated ram to 4gb, that will leave 2gb for the os and other stuff.
If you still have issues, post new log.