Add extra guarding against legacy widget preview errors (#44635)

* Add extra guarding against legacy widget preview errors

* Add a fallback height

packages/widgets/src/blocks/legacy-widget/edit/preview.js

@@ -61,10 +61,15 @@ export default function Preview( { idBase, instance, isVisible } ) {
 			function setHeight() {
 				// Pick the maximum of these two values to account for margin collapsing.
 				const height = Math.max(
-					iframe.contentDocument.documentElement.offsetHeight,
-					iframe.contentDocument.body.offsetHeight
+					iframe.contentDocument.documentElement?.offsetHeight ?? 0,
+					iframe.contentDocument.body?.offsetHeight ?? 0
 				);
-				iframe.style.height = `${ height }px`;
+
+				// Fallback to a height of 100px if the height cannot be determined.
+				// This ensures the block is still selectable. 100px should hopefully
+				// be not so big that it's annoying, and not so small that nothing
+				// can be seen.
+				iframe.style.height = `${ height !== 0 ? height : 100 }px`;
 			}
 
 			const { IntersectionObserver } = iframe.ownerDocument.defaultView;