Adding missing break in plugin validation check (#43180)

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 759734c..a2a8e59 100644
--- a/packages/flutter_tools/lib/src/flutter_manifest.dart
+++ b/packages/flutter_tools/lib/src/flutter_manifest.dart
@@ -401,8 +401,9 @@
         }
         break;
       case 'plugin':
-        if (kvp.value is! YamlMap) {
+        if (kvp.value is! YamlMap || kvp.value == null) {
           errors.add('Expected "${kvp.key}" to be an object, but got ${kvp.value} (${kvp.value.runtimeType}).');
+          break;
         }
         final List<String> pluginErrors = Plugin.validatePluginYaml(kvp.value);
         errors.addAll(pluginErrors);