_cacheExistingSharedStrings() {
        this._node.children.forEach((node, i) => {
            const content = node.children[0];
            if (content.name === "t") {
                const string = content.children[0];
                this._stringArray.push(string);
                this._indexMap[string] = i;
            } else {
                // TODO: Properly support rich text nodes in the future. For now just store the object as a placeholder.
                this._stringArray.push(node.children);
                this._indexMap[JSON.stringify(node.children)] = i;
            }
        });
    }
Store any existing values in the caches.
@private
@returns {undefined}