defaultFillColor Error thrown inconsistently.

In my opinion, looping is a better way because you can carefully curate each item based on its properties. If something goes wrong, you can look at the object that broke it, and adjust your logic to exclude/handle it.

I've written a script to get started. It is very simple in terms of the logic but I think it can handle many cases. If you decide to try it out, let me know how it goes.

- Mark

/**
 * Make a CompoundPathItem out of document's path items
 * ONLY IF they all have the same fillColor value.
 * Note: this will work even if the items are locked or hidden.
 * @discussion https://community.adobe.com/t5/illustrator-discussions/defaultfillcolor-error-thrown-inconsistently/m-p/13651215
 */
(function () {

    var doc = app.activeDocument;

    // only PathItems can go into CompoundPathItems
    var items = doc.pathItems,
        uniformItems = getUniformFillColoredPathItems(items);

    if (
        uniformItems == undefined
        || uniformItems.length < 1
    ) {
        alert('Document doesn\'t have uniformly-colored path items.');
        return;
    }

    // make a compoundPathItem out of the uniformItems
    var cp = doc.compoundPathItems.add();
    for (var i = uniformItems.length - 1; i >= 0; i--)
        uniformItems[i].move(cp, ElementPlacement.PLACEATBEGINNING);

})();


/**
 * Will return an array of items ONLY IF those
 * items all have the same fillColor value.
 * Note: items without a fillColor property
 * are ignored.
 *  m1b
 *  2023-03-15
 *  {Array<PageItem>} items - the items to check.
 *  {Array<PathItem>} - the uniform-colored items.
 */
function getUniformFillColoredPathItems(items) {

    var uniformFillColoredItems = [],
        theFillColor;

    for (var i = 0; i < items.length; i++) {

        var item = items[i];

        if (item.typename !== 'PathItem')
            continue;

        if (theFillColor == undefined) {

            if (
                item.fillColor != undefined
            ) {
                theFillColor = item.fillColor;
                uniformFillColoredItems.push(item);
            }
            else {
                continue;
            }

        }

        else {
            if (objectsAreEqual(theFillColor, item.fillColor))
                // found matching fillColor
                uniformFillColoredItems.push(item);
            else
                // this one didn't match so game over
                return;
        }

    }

    return uniformFillColoredItems;

};


/**
 * Returns true if the two objects have
 * matching values for each property.
 *  m1b
 *  2023-03-15
 *  {Object} obj1 - an object to compare.
 *  {Object} obj2 - an object to compare.
 *  {Boolean}
 */
function objectsAreEqual(obj1, obj2) {
    for (key in obj1) {
        if (
            obj1.hasOwnProperty(key)
            && obj1[key] != obj2[key]
        )
            return false;
    }
    return true;
};

P.S. this script is fairly strict about whether color A matches color B. You may want to make it less strict, such comparing the color channel values.