If an entry in a data collection group config file contains malformed XML (in my case, the mibObj was missing an alias attribute), the config-tester doesn't catch it and allows ONMS to start. Collectd then throws an NPE and data collection nearly silently fails with little indication as to the cause of the NPE. xmllint does catch this issue, FWIW.

A test config is attached that should fail due to the missing alias attribute on line 34.