fix: throw exception if flavor not found in shorebird.yaml

packages/flutter_tools/gradle/src/main/groovy/flutter.groovy

@@ -1404,7 +1404,11 @@ class FlutterPlugin implements Plugin<Project> {
 
                   // Update the app_id to the correct flavor if one was provided.
                   if (variant.flavorName != null && !variant.flavorName.isEmpty()) {
-                    shorebirdYamlData['app_id'] = shorebirdYamlData.flavors[variant.flavorName]
+                    def shorebirdFlavor = shorebirdYamlData.flavors[variant.flavorName]
+                    if (shorebirdFlavor == null) {
+                      throw new GradleException("Flavor '${variant.flavorName}' not found in shorebird.yaml")
+                    }
+                    shorebirdYamlData['app_id'] = shorebirdFlavor
                   }
 
                   // Remove any flavors. This is a no-op if the flavors key doesn't exist.

packages/flutter_tools/test/general.shard/shorebird/shorebird_yaml_test.dart

@@ -85,7 +85,16 @@ base_url: https://example.com
       final Directory tempDir = Directory.systemTemp.createTempSync('shorebird_yaml_test.');
       final File tempFile = File('${tempDir.path}/shorebird.yaml');
       tempFile.writeAsStringSync(yamlContents);
-      updateShorebirdYaml(const BuildInfo(BuildMode.release, 'foo', treeShakeIcons: false), tempFile.path, environment: <String, String>{'SHOREBIRD_PUBLIC_KEY': '4-a'});
+      updateShorebirdYaml(
+        const BuildInfo(
+          BuildMode.release,
+          'foo',
+          treeShakeIcons: false,
+          packageConfigPath: '',
+        ),
+        tempFile.path,
+        environment: <String, String>{'SHOREBIRD_PUBLIC_KEY': '4-a'},
+      );
       final String updatedContents = tempFile.readAsStringSync();
       // Order is not guaranteed, so parse as YAML to compare.
       final YamlDocument updated = loadYamlDocument(updatedContents);