That error has only been reported one other time that I am aware of, and it turned out to be a corrupted alerts.log file if I recall correctly.
That error is saying that a value read from the alerts.log was NULL, but there is no way that can happen unless the file is corrupt. One possibility is a disk read or write error.
When you go to the ALERTS tab, can you display all the alerts from all Suricata configured interfaces without any problem? The exact same code is used within the ALERTS tab and Suricata Wiget, so a corrupt file would impact both. It's also possible that the error happened some time back in the past and by now the alerts.log has been rotated out and the current file may be fine. Does the error repeat every single time the widget updates, or has it only popped up once?
Normally, issues with Snort or Suricata would be posted here: https://forum.netgate.com/category/53/ids-ips.