You most likely have something other than "Load Nothing" (as I have selected) in the startup options drop down. Change it to load nothing, or fix the configuration in DSDPlus.siteLoader and that error will most likely go away.
Sounds like DSD+ is being told to load a specific network/site, but there is no frequency data for that site in the data files. How one does that depends on what type of trunking system it is.