Fix NPE thrown when creating a default transfer manager (#2726)

* Fix NPE thrown when creating a default transfer manager

* rename builder

Author: zoewangg

.changes/next-release/bugfix-S3TransferManager-1dee8e5.json

@@ -0,0 +1,6 @@
+{
+    "category": "S3 Transfer Manager", 
+    "contributor": "", 
+    "type": "bugfix", 
+    "description": "Fix the NPE thrown when calling `S3TransferManager.create()`"
+}

services-custom/s3-transfer-manager/src/main/java/software/amazon/awssdk/transfer/s3/internal/DefaultS3TransferManager.java

@@ -37,14 +37,16 @@
 public final class DefaultS3TransferManager implements S3TransferManager {
     private final S3CrtAsyncClient s3CrtAsyncClient;
 
-    public DefaultS3TransferManager(DefaultBuilder builder) {
+    public DefaultS3TransferManager(DefaultBuilder tmBuilder) {
         S3CrtAsyncClient.S3CrtAsyncClientBuilder clientBuilder = S3CrtAsyncClient.builder();
-        builder.s3ClientConfiguration.credentialsProvider().ifPresent(clientBuilder::credentialsProvider);
-        builder.s3ClientConfiguration.maxConcurrency().ifPresent(clientBuilder::maxConcurrency);
-        builder.s3ClientConfiguration.minimumPartSizeInBytes().ifPresent(clientBuilder::minimumPartSizeInBytes);
-        builder.s3ClientConfiguration.region().ifPresent(clientBuilder::region);
-        builder.s3ClientConfiguration.targetThroughputInGbps().ifPresent(clientBuilder::targetThroughputInGbps);
-        builder.s3ClientConfiguration.asyncConfiguration().ifPresent(clientBuilder::asyncConfiguration);
+        if (tmBuilder.s3ClientConfiguration != null) {
+            tmBuilder.s3ClientConfiguration.credentialsProvider().ifPresent(clientBuilder::credentialsProvider);
+            tmBuilder.s3ClientConfiguration.maxConcurrency().ifPresent(clientBuilder::maxConcurrency);
+            tmBuilder.s3ClientConfiguration.minimumPartSizeInBytes().ifPresent(clientBuilder::minimumPartSizeInBytes);
+            tmBuilder.s3ClientConfiguration.region().ifPresent(clientBuilder::region);
+            tmBuilder.s3ClientConfiguration.targetThroughputInGbps().ifPresent(clientBuilder::targetThroughputInGbps);
+            tmBuilder.s3ClientConfiguration.asyncConfiguration().ifPresent(clientBuilder::asyncConfiguration);
+        }
 
         s3CrtAsyncClient = clientBuilder.build();
     }

services-custom/s3-transfer-manager/src/test/java/software/amazon/awssdk/transfer/s3/internal/S3TransferManagerTest.java

@@ -42,9 +42,6 @@ public class S3TransferManagerTest {
     private S3CrtAsyncClient mockS3Crt;
     private S3TransferManager tm;
 
-    @Rule
-    public ExpectedException thrown = ExpectedException.none();
-
     @Before
     public void methodSetup() {
         mockS3Crt = mock(S3CrtAsyncClient.class);
@@ -56,6 +53,12 @@ public void methodTeardown() {
         tm.close();
     }
 
+    @Test
+    public void defaultTransferManager_shouldNotThrowException() {
+        S3TransferManager transferManager = S3TransferManager.create();
+        transferManager.close();
+    }
+
     @Test
     public void upload_returnsResponse() {
         PutObjectResponse response = PutObjectResponse.builder().build();