XML import "Nullable object must have a value"

Just letting people know of a solution to a recent problem.

If you get an error importing an xml file, and the log file includes "Nullable object must have a value" and a reference to "calendar", then the error is likely because you have not set a default calendar. The log file does not really provide any specific information to suggest this.

To resolve go to calendars and check one of the global calendars to use as default, prior to importing.

In my case this came about because I updated P6 version just prior to importing xml, and in this recently updated state no calendar is set as default.