Fix NPE when traversing map.

We now use regular iteration instead of the Stream API.

Closes: #4567
Original pull request: #4568

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java

@@ -617,15 +617,20 @@ protected Object convertSimpleOrDocument(Object source, @Nullable MongoPersisten
 
 		if (source instanceof Map<?,?> sourceMap) {
 
-			return sourceMap.entrySet().stream().collect(Collectors.toMap(
-					entry -> ObjectUtils.nullSafeToString(converter.convertToMongoType(entry.getKey())),
-					entry -> {
-						if (entry.getValue() instanceof Document document) {
-							return getMappedObject(document, entity);
-						}
-						return delegateConvertToMongoType(entry.getValue(), entity);
-					}
-			));
+			Map<String, Object> map = new LinkedHashMap<>(sourceMap.size(), 1F);
+
+			sourceMap.entrySet().forEach(it -> {
+
+				String key = ObjectUtils.nullSafeToString(converter.convertToMongoType(it.getKey()));
+
+				if (it.getValue() instanceof Document document) {
+					map.put(key, getMappedObject(document, entity));
+				} else {
+					map.put(key, delegateConvertToMongoType(it.getValue(), entity));
+				}
+			});
+
+			return map;
 		}
 
 		return delegateConvertToMongoType(source, entity);

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/UpdateMapperUnitTests.java

@@ -762,6 +762,18 @@ void mappingShouldNotContainTypeInformationWhenValueTypeOfMapMatchesDeclaration(
 		assertThat(mappedUpdate).doesNotContainKey("$set.concreteMap.jasnah._class");
 	}
 
+	@Test // GH-4567
+	void updateShouldAllowNullValuesInMap() {
+
+		Map<Object, NestedDocument> map = Collections.singletonMap("jasnah", new NestedDocument("kholin"));
+
+		Update update = new Update().set("concreteMap", Collections.singletonMap("jasnah", null));
+		Document mappedUpdate = mapper.getMappedObject(update.getUpdateObject(),
+				context.getPersistentEntity(EntityWithObjectMap.class));
+
+		assertThat(mappedUpdate).isEqualTo(new Document("$set", new Document("concreteMap", Collections.singletonMap("jasnah", null))));
+	}
+
 	@Test // DATAMONGO-1250
 	@SuppressWarnings("unchecked")
 	void mapsUpdateWithBothReadingAndWritingConverterRegistered() {