I have a similar problem, though my 2018 MacBook Air does not reboot. I find if you have anything in your clipboard and you go to select text in any app, the app crashes. I've sent a feedback report with the crash logs to Apple. Hopefuly the next beta is around the corner.
Edit: Just reproduced without clipboard data. Select text once and you're ok. Select text a second time it crashes.
Crash Reporter shows ;
Exception Type: EXC_BAD_INSTRUCTION (SIGILL)
Exception Codes: 0x0000000000000001, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Termination Signal: Illegal instruction: 4
Termination Reason: Namespace SIGNAL, Code 0x4
Terminating Process: exc handler 
Application Specific Information:
XPC API Misuse: Release of last reference on a suspended connection.
Application Specific Signatures:
It seems the worst culprits are the Microsoftsoft Office 365 suite (especially Word and Outlook). Sometime just choosing a menu option causes this to happen let alone selecting text.
I now have to use Virtual Box to run the Windows version of Word and Outlook to get stuff done which is a tad annoying, but this the joys of a beta. Cutting edge and bleeding edge are frequently the same thing.
Regarding the spin, if you leave the computer for a couple of minutes once it freezes, it will then show the black screen of death and automatically restart.