I have the same version (0.99.30-4) of software-properties-gtk
installed.  I ran across this same bug while trying to figure out why
Synaptic is not able to open its window that manages repositories
(instead a dialog box appears that says "The repository information has
changed. You have to click on the "Reload" button for your changes to
take effect", but clicking either Reload or Cancel has no effect on the
problem).  Launching Synaptic from a terminal and then once again trying
to open Repositories from the Settings menu produces error information
in the terminal window that matches the info given in the original
poster's report.