made suffix nullable

pkgs/yaml/lib/src/parser.dart

diff --git a/pkgs/yaml/lib/src/parser.dart b/pkgs/yaml/lib/src/parser.dart
index 267d7ca..0f824be 100644
--- a/pkgs/yaml/lib/src/parser.dart
+++ b/pkgs/yaml/lib/src/parser.dart
@@ -287,7 +287,7 @@
           throw YamlException('Undefined tag handle.', tagToken!.span);
         }
 
-        tag = tagDirective.prefix + tagToken!.suffix;
+        tag = tagDirective.prefix + (tagToken?.suffix ?? '');
       }
     }
 

pkgs/yaml/lib/src/token.dart

diff --git a/pkgs/yaml/lib/src/token.dart b/pkgs/yaml/lib/src/token.dart
index 4f4172f..447e979 100644
--- a/pkgs/yaml/lib/src/token.dart
+++ b/pkgs/yaml/lib/src/token.dart
@@ -96,7 +96,7 @@
   final String? handle;
 
   /// The tag suffix, or `null`.
-  final String suffix;
+  final String? suffix;
 
   TagToken(this.span, this.handle, this.suffix);