Bug 1663636 - Prevent trying to fetch font faces of destroyed node fronts. r=jdescottes

This isn't really related to EventLoop, but this is another exception thrown when running DAMP.
This time related to a request made against an already destroyed NodeFront.

Differential Revision: https://phabricator.services.mozilla.com/D89451

devtools/client/inspector/rules/models/element-style.js

@@ -219,6 +219,10 @@ class ElementStyle {
   getUsedFontFamilies() {
     return new Promise((resolve, reject) => {
       this.ruleView.styleWindow.requestIdleCallback(async () => {
+        if (this.element.isDestroyed()) {
+          resolve([]);
+          return;
+        }
         try {
           const fonts = await this.pageStyle.getUsedFontFaces(this.element, {
             includePreviews: false,