[go_router] Fix patternToPath to throw when path parameter is missing

packages/go_router/lib/src/path_utils.dart

@@ -85,7 +85,11 @@ String patternToPath(String pattern, Map<String, String> pathParameters) {
       buffer.write(pattern.substring(start, match.start));
     }
     final String name = match[1]!;
-    buffer.write(pathParameters[name]);
+    final String? value = pathParameters[name];
+    if (value == null) {
+      throw GoException('Missing path parameter: $name for pattern "$pattern"');
+    }
+    buffer.write(value);
     start = match.end;
   }
 

packages/go_router/pending_changelogs/fix_pattern_to_path_throws.yaml

@@ -0,0 +1,3 @@
+changelog: |
+  - Fixes `patternToPath` to throw `GoException` when a path parameter is missing instead of producing URLs containing the literal "null".
+version: patch

packages/go_router/test/path_utils_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter_test/flutter_test.dart';
+import 'package:go_router/go_router.dart' show GoException;
 import 'package:go_router/src/path_utils.dart';
 
 void main() {
@@ -95,6 +96,19 @@ void main() {
     expect(url, restoredUrl);
   });
 
+  test('patternToPath throws when path parameter is missing', () {
+    const pattern = '/user/:id/book/:bookId';
+    final incompleteParams = <String, String>{'id': '123'};
+    expect(
+      () => patternToPath(pattern, incompleteParams),
+      throwsA(isA<GoException>().having(
+        (GoException e) => e.message,
+        'message',
+        contains('Missing path parameter: bookId'),
+      )),
+    );
+  });
+
   test('concatenatePaths', () {
     void verify(String pathA, String pathB, String expected) {
       final String result = concatenatePaths(pathA, pathB);