I'm using the radiant version that came with RC6, I assume its gtk radiant, but the "about" page calls it both "DOOM radiant 1.0" and "Q3radiant"
whatever version it was installed along with the ufoai-2.0-RC6-win32.exe install file that was online on December 23rd. Anyway there is nothing under plugin other than "refresh".
Though I've had a feeling it was the worldspawn. I first made the worldspawn manually, then when it produced the error, copied the worldspawn from the dam map, but to no avail.