Linux Mint VM crashes when MacOS sleeps?
This is expected behavior.

Your VM is not going to continue to run when your Mac goes to sleep.  In this case, your VirtualBox will attempt to save the state of the machine.  I have found FreeBSD to be forgiving in these cases, but Linux (Ubuntu) has never liked it when the Mac went to sleep and VB attempted to save the state.

The most effective solution is to disable sleep when your VM is running.