Error log showing "null: no thrown error" when loading certain Dashboard(s)

In some cases, the localhost logs may show: "SEVERE *** ERROR *** ClassUtil - class not found: null"

The reason for the error could be that there were records on sys_grid_canvas_pane table which had either no Portal Widgets associated or had a sys_id for which there was no corresponding sys_portal record (invalid references). Following URL gives you a list of all such records:
https://<instance>.service-now.com/sys_grid_canvas_pane_list.do?sysparm_query=portal_widget.sys_idISEMPTY

This would typically happen when the dashboard is developed on one instance and then moved to another instance in a way which does not completely moves the configuration.

The documentation for moving dashboards via update set has below warnings.

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This procedure does not move the content of the dashboard (such as reports, PA widgets, content blocks, and other widgets) to the update set. You can add dashboard content to the update set using default platform functionality.

Note: Make sure that dashboard content is in either in the same update set as the dashboard record or is already present in the target instance. Errors result if the moved dashboard points to content that does not exist on the target instance.

(Required) On the target instance, make sure that all the tabs of the dashboard have associated portal pages.
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Here is the documentation link.

https://docs.servicenow.com/bundle/newyork-performance-analytics-and-reporting/page/use/performance-analytics/task/t_MoveDashboardWithUpdateSet.html

Following steps can be taken to avoid these errors in the system logs:

1. Ensure that all the objects used in the dashboard (such as reports, PA widgets, content blocks, and other widgets) are brought over to the target instance.
2. If some error still show up after bringing across all the configurations and the dashboard itself is not having any issues except for the errors appearing in the logs, deleting all the sys_grid_canvas_pane records having invalid references returned by the following URL should get rid of the errors in the logs.

https://<instance>.service-now.com/sys_grid_canvas_pane_list.do?sysparm_query=portal_widget.sys_idISEMPTY

Note:

1. Take an XML backup of the records before deleting them so that you can revert in case of any issues.
2. Please test the solution on a recently cloned instance before applying any solution on the production instance.