fix(batch): guard against null reference

Author: bevzzz

src/main/java/io/weaviate/client6/v1/api/collections/batch/BatchContext.java

@@ -363,7 +363,9 @@ private void shutdown() {
         queue.put(TaskHandle.POISON);
 
         // Wait for both "send" to exit; "send" will not exit until "recv" completes.
-        send.get();
+        if (send != null) {
+          send.get();
+        }
         closing.complete(null);
       } catch (Exception e) {
         closing.completeExceptionally(e);
@@ -378,7 +380,9 @@ private void shutdownNow(Exception ex) {
     messages.onError(Status.INTERNAL.withCause(ex).asRuntimeException());
 
     // Terminate the "send" routine.
-    send.cancel(true);
+    if (send != null) {
+      send.cancel(true);
+    }
 
     if (!closed) {
       // Since shutdownNow is never triggered by the "main" thread,

src/test/java/io/weaviate/client6/v1/api/collections/batch/BatchContextTest.java

@@ -16,6 +16,8 @@
 import java.util.concurrent.TimeUnit;
 import java.util.stream.Stream;
 
+import javax.annotation.concurrent.GuardedBy;
+
 import org.assertj.core.api.Assertions;
 import org.junit.After;
 import org.junit.AfterClass;
@@ -75,6 +77,7 @@ public class BatchContextTest {
       1);
 
   /** Batch context for the current test case. */
+  @GuardedBy("this")
   private BatchContext<Map<String, Object>> context;
   /** Server half of the stream. */
   private volatile OutboundStream out;
@@ -92,7 +95,8 @@ private StreamObserver<Message> createStream(StreamObserver<Event> recv) {
    * descriptor, and collection handle defaults.
    */
   @Before
-  public void startContext() {
+  public synchronized void startContext() {
+    assert context == null;
     context = new BatchContext.Builder<>(this::createStream, MAX_SIZE_BYTES, DESCRIPTOR, DEFAULTS)
         .batchSize(BATCH_SIZE)
         .queueSize(QUEUE_SIZE)
@@ -102,7 +106,7 @@ public void startContext() {
   }
 
   @After
-  public void reset() throws Exception {
+  public synchronized void reset() throws Exception {
     if (context != null) {
       // Some of the tests may close the context, so this
       // implicitly tests that closing it multiple times is OK.