FIX: ensures we have an elementFromPoint before using it (#7583)

app/assets/javascripts/discourse/components/scrolling-post-stream.js.es6

@@ -72,7 +72,11 @@ export default MountWidget.extend({
 
     // We use this because watching videos fullscreen in Chrome was super buggy
     // otherwise. Thanks to arrendek from q23 for the technique.
-    if (document.elementFromPoint(0, 0).tagName.toUpperCase() === "IFRAME") {
+    const topLeftCornerElement = document.elementFromPoint(0, 0);
+    if (
+      topLeftCornerElement &&
+      topLeftCornerElement.tagName.toUpperCase() === "IFRAME"
+    ) {
       return;
     }