Nagvis - The must value "host_name" is not set

When working in Nagvis you get an error

The must value "host_name" is not set in an object of type "service" in map "mapname" 
(note: can be service, host, etc)

This is caused when you accidentally modify an object and it removes required fields in the config file.

To fix, go to your maps config file.  In opsview it's located at /usr/local/nagios/nagvis/etc/map

nano mapname.cfg
remove the offending entry.  In my case you can see that it defines a service with no contents.