Handle case where `nodeFromXPath` returns a non-Element result

Handle scenarios flagged by type checking where `nodeFromXPath` may
return `null` or a non-Element node. Previously it could never return
`null` but would throw an exception if the node was not found. In the
new code a check is needed.

The case where `nodeFromXPath` returned a non-Element node would never
happen with XPaths generated by the client but could happen if an
annotation was created via the API with a non-simple XPath.

src/annotator/anchoring/range.js

@@ -380,6 +380,9 @@ export class SerializedRange {
       let node;
       try {
         node = nodeFromXPath(this[p], root);
+        if (!node) {
+          throw new Error('Node not found');
+        }
       } catch (e) {
         throw new RangeError(`Error while finding ${p} node: ${this[p]}: ` + e);
       }

src/annotator/anchoring/xpath-util.js

@@ -62,14 +62,14 @@ export function xpathFromNode(node, root) {
 }
 
 /**
- * Return all text node descendants of `element`.
+ * Return all text node descendants of `parent`.
  *
- * @param {Element} element
+ * @param {Node} parent
  * @return {Text[]}
  */
-export function getTextNodes(element) {
+export function getTextNodes(parent) {
   const nodes = [];
-  for (let node of Array.from(element.childNodes)) {
+  for (let node of Array.from(parent.childNodes)) {
     // We test `nodeType` here rather than using `instanceof` because we have
     // tests where `node` comes from a different iframe.
     if (node.nodeType === Node.TEXT_NODE) {

src/annotator/anchoring/xpath.js

@@ -97,6 +97,7 @@ function evaluateSimpleXPath(xpath, root) {
  *
  * @param {string} xpath
  * @param {Element} [root]
+ * @return {Node|null}
  */
 export function nodeFromXPath(xpath, root = document.body) {
   try {