whoopsie crashes on report permissions error

When a file in /var/crash is not writeable by apport, whoopsie crashes. This caused my laptop to go into an infinite loop on rebooting after upgrading to focal. (Was this because apport was trying to report a bug in itself?)

Feb 19 16:33:35 aspidistra whoopsie-upload-all[17941]: Collecting info for /var/crash/_usr_bin_Xwayland.120.crash...
Feb 19 16:33:35 aspidistra whoopsie-upload-all[17941]: Traceback (most recent call last):
Feb 19 16:33:35 aspidistra whoopsie-upload-all[17941]: File "/usr/share/apport/whoopsie-upload-all", line 162, in <module>
Feb 19 16:33:35 aspidistra whoopsie-upload-all[17941]: stamps = collect_info()
Feb 19 16:33:35 aspidistra whoopsie-upload-all[17941]: File "/usr/share/apport/whoopsie-upload-all", line 112, in collect_info
Feb 19 16:33:35 aspidistra whoopsie-upload-all[17941]: res = process_report(r)
Feb 19 16:33:35 aspidistra whoopsie-upload-all[17941]: File "/usr/share/apport/whoopsie-upload-all", line 87, in process_report
Feb 19 16:33:35 aspidistra whoopsie-upload-all[17941]: with open(report, 'ab') as f:
Feb 19 16:33:35 aspidistra whoopsie-upload-all[17941]: PermissionError: [Errno 13] Permission denied: '/var/crash/_usr_bin_Xwayland.120.crash'
Feb 19 16:33:35 aspidistra systemd[1]: apport-autoreport.service: Main process exited, code=exited, status=1/FAILURE
Feb 19 16:33:35 aspidistra systemd[1]: apport-autoreport.service: Failed with result 'exit-code'.
Feb 19 16:33:35 aspidistra systemd[1]: Failed to start Process error reports when automatic reporting is enabled.
Feb 19 16:33:35 aspidistra systemd[1]: Starting Process error reports when automatic reporting is enabled...

and so on. /var/crash/_usr_bin_Xwayland.120.crash was owned by gdm, group whoopsie, and had permissions 640. Deleting the file (in recovery mode) allowed the boot to finish successfully.