protected String getErrorString() {
   for (MappingPriorityRule rule : myProperties.getPriorityRules()) {
     if (rule.getLeft().isIncomplete() || rule.getRight().isIncomplete()) {
       return "One or more priority rules are incomplete";
     }
   }
   GeneratorDescriptor tmpDescr = new GeneratorDescriptor();
   myProperties.saveTo(tmpDescr);
   return null;
 }