added error handling for invalid urls

lib/pages/collection_page.dart

@@ -45,8 +45,9 @@ class _CollectionPageState extends State<CollectionPage> {
       selectedBookmark: selectedBookmark,
       onSavePressed: onBookmarkSaved,
       onDeletePressed: () {
-        Storage.deleteBookmarkById(selectedBookmark.id);
-        setState(() {});
+        Storage.deleteBookmarkById(
+          selectedBookmark.id,
+        ).whenComplete(() => setState(() {}));
       },
     ),
   );
@@ -60,7 +61,11 @@ class _CollectionPageState extends State<CollectionPage> {
   Widget bookmarksListItemBuilder(BuildContext context, Bookmark bookmark) {
     return ListTile(
       title: Text(bookmark.name),
-      onTap: () => launchUrlFromString(bookmark.link),
+      onTap: () => launchUrlFromString(bookmark.link).then((errorCode) {
+        if (context.mounted && errorCode != UrlLaunchErrorCode.none) {
+          return showUrlError(context, errorCode);
+        }
+      }),
       onLongPress: () => editBookmark(bookmark),
     );
   }
@@ -104,4 +109,16 @@ class _CollectionPageState extends State<CollectionPage> {
       ),
     );
   }
+
+  void showUrlError(BuildContext context, UrlLaunchErrorCode errorCode) {
+    String errorText = '';
+    if (errorCode == UrlLaunchErrorCode.couldNotLaunch) {
+      errorText = 'Could not launch Url';
+    } else {
+      errorText = 'Invalid Url';
+    }
+    ScaffoldMessenger.of(
+      context,
+    ).showSnackBar(SnackBar(content: Text(errorText)));
+  }
 }