Fix a Text Selection Exception in Safari (WordPress#8255)

* Catch a Text Selection Exception in Safari; fixes WordPress#8254

* Improve the inline explanation of why we need a try/catch block

packages/dom/src/dom.js

@@ -433,13 +433,25 @@ export function placeCaretAtVerticalEdge( container, isReverse, rect, mayUseScro
  * @return {boolean} True if the element is an text field, false if not.
  */
 export function isTextField( element ) {
-	const { nodeName, selectionStart, contentEditable } = element;
-
-	return (
-		( nodeName === 'INPUT' && selectionStart !== null ) ||
-		( nodeName === 'TEXTAREA' ) ||
-		contentEditable === 'true'
-	);
+	try {
+		const { nodeName, selectionStart, contentEditable } = element;
+
+		return (
+			( nodeName === 'INPUT' && selectionStart !== null ) ||
+			( nodeName === 'TEXTAREA' ) ||
+			contentEditable === 'true'
+		);
+	} catch ( error ) {
+		// Safari throws an exception when trying to get `selectionStart`
+		// on non-text <input> elements (which, understandably, don't
+		// have the text selection API). We catch this via a try/catch
+		// block, as opposed to a more explicit check of the element's
+		// input types, because of Safari's non-standard behavior. This
+		// also means we don't have to worry about the list of input
+		// types that support `selectionStart` changing as the HTML spec
+		// evolves over time.
+		return false;
+	}
 }
 
 /**