Fix #31764: Show appropriate error message when fonts pubspec.yaml isn't iterable

Show appropriate error message when fonts isn't iterable

packages/flutter_tools/lib/src/flutter_manifest.dart

diff --git a/packages/flutter_tools/lib/src/flutter_manifest.dart b/packages/flutter_tools/lib/src/flutter_manifest.dart
index 5eb94d1..a757486 100644
--- a/packages/flutter_tools/lib/src/flutter_manifest.dart
+++ b/packages/flutter_tools/lib/src/flutter_manifest.dart
@@ -398,11 +398,11 @@
     if (fontMap['family'] != null && fontMap['family'] is! String) {
       errors.add('Font family must either be null or a String.');
     }
-    if (fontMap['fonts'] != null && fontMap['fonts'] is! YamlList) {
-      errors.add('Expected "fonts" to either be null or a list.');
-    }
     if (fontMap['fonts'] == null) {
       continue;
+    } else if (fontMap['fonts'] is! YamlList) {
+      errors.add('Expected "fonts" to either be null or a list.');
+      continue;
     }
     for (final YamlMap fontListItem in fontMap['fonts']) {
       for (final MapEntry<dynamic, dynamic> kvp in fontListItem.entries) {