MacBook Pro crashes every night

Every night my MacBook Pro shuts down, and I get the error message when I power up the following morning:

panic(cpu 0 caller 0xffffff8001e89d2a): Sleep transition timed out after 180 seconds while calling power state change callbacks. Suspected bundle: com.apple.driver.usb.AppleUSBXHCI. Thread 0x3f6c4.

I've read a popular thread here that it's to do with connection to an external monitor, but I am not using an external monitor. I have also changed Notifications settings to Turn on Do Not Disturb for all 4 choices (time, display sleeping, screen locked, and mirroring), as suggested. But none of these things have worked.

I think it started happening when I upgraded to Big Sur 11.2.1.

Any help / suggestions gratefully accepted.