code: function canTotalBeCalculatedAndIfNotWarnUser(item) {
    if (canResultsBeSummed(item)) {
      return true;
    }
    var conceptItems = item._concepts[0].items;
    var changedActive = [];
    conceptItems.forEach(concept => {
      var numberActive = concept.items.filter(function(subconcept) {
        return subconcept.isActive;
      }).length;
      if (numberActive > 1) {
        changedActive.push('"' + concept.name + '"');
      }
    });
    if (changedActive.length > 0) {
      item.terria.error.raiseEvent(
        new TerriaError({
          sender: item,
          title: "Cannot calculate a total",
          message:
            "You have selected multiple values for " +
            changedActive.join(" and ") +
            ", but the measure you now have chosen cannot be totalled across them. \
              As a result, there is no obvious measure to use to shade the regions (although you can still choose a region to view its data).\
              To see the regions shaded again, please select only one value for " +
            changedActive.join(" and ") +
            ", or select a different measure."
        })
      );
      return false;
    }
    return true;
  }
docs: Only show a warning if more than one value of a concept has been selected. Returns true if the user has been warned.