Improve robustness of art asset lookup...if an asset is not found, issue a...

Improve robustness of art asset lookup...if an asset is not found, issue a console error and move on instead of crashing hard.

js/artInfrastructure.js

@@ -60,6 +60,10 @@ App.Art.SvgQueue = class {
 	add(id) {
 		const res = this._cache.get(id);
 		let clones = [];
+		if (!res) {
+			console.error(`Missing art resource: ${id}`);
+			return;
+		}
 		for (const srcNode of res.children) {
 			/** @type {Element} */
 			const node = srcNode.cloneNode(true);