add null check for custom style builder

app/lib/generated_plugin_registrant.dart

@@ -1,15 +0,0 @@
-//
-// Generated file. Do not edit.
-//
-
-// ignore_for_file: lines_longer_than_80_chars
-
-import 'package:url_launcher_web/url_launcher_web.dart';
-
-import 'package:flutter_web_plugins/flutter_web_plugins.dart';
-
-// ignore: public_member_api_docs
-void registerPlugins(Registrar registrar) {
-  UrlLauncherPlugin.registerWith(registrar);
-  registrar.registerMessageHandler();
-}

app/windows/flutter/ephemeral/.plugin_symlinks/url_launcher_windows

@@ -1 +0,0 @@
-C:/flutter/.pub-cache/hosted/pub.dartlang.org/url_launcher_windows-0.0.1+3/

lib/src/widgets/text_line.dart

@@ -246,7 +246,7 @@ class TextLine extends StatelessWidget {
           ///Unkown Attribute
           ///
           // ignore: lines_longer_than_80_chars
-          final customStyle = styleBuilder?.call(textNode.style.attributes[key]!);
+          final customStyle = styleBuilder?.call(textNode.style.attributes[key]);
           if (customStyle != null) res = res.merge(customStyle);
         }
       });