The visualization references a Master Visualization that is not present in the master library. Object 'xxx' claims to extend nonexsistent object 'yyy'.
A sheet object links to a Master Visualization that is not present in the Master Library.
This condition can occur when a developer removes a Master Visualization from the library without realizing the Visualization is in use.
Because Master items are referenced by id only, the original type or title the missing Visualization are unknown.