Fix NPE when reading/mapping null value inside collection.

Closes: #3686

Author: christophstrobl

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

@@ -1269,7 +1269,7 @@ protected Object readCollectionOrArray(ConversionContext context, Collection<?>
 		}
 
 		for (Object element : source) {
-			items.add(context.convert(element, componentType));
+			items.add(element != null ? context.convert(element, componentType) : element);
 		}
 
 		return getPotentiallyConvertedSimpleRead(items, targetType.getType());
@@ -2013,6 +2013,7 @@ protected static class ConversionContext {
 		@SuppressWarnings("unchecked")
 		public <S extends Object> S convert(Object source, TypeInformation<? extends S> typeHint) {
 
+			Assert.notNull(source, "Source must not be null");
 			Assert.notNull(typeHint, "TypeInformation must not be null");
 
 			if (conversions.hasCustomReadTarget(source.getClass(), typeHint.getType())) {

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

@@ -110,7 +110,8 @@ public Object readReference(MongoPersistentProperty property, Object value, Look
 			return null;
 		}
 
-		return entityReader.read(result.iterator().next(), property.getTypeInformation());
+		Object resultValue = result.iterator().next();
+		return resultValue != null ? entityReader.read(resultValue, property.getTypeInformation()) : null;
 	}
 
 	private ReferenceCollection computeReferenceContext(MongoPersistentProperty property, Object value,

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

@@ -2532,6 +2532,41 @@ void shouldWriteNullPropertyCorrectly() {
 		assertThat(document).containsEntry("writeAlwaysPerson", null).doesNotContainKey("writeNonNullPerson");
 	}
 
+	@Test // GH-3686
+	void readsCollectionContainingNullValue() {
+
+		org.bson.Document source = new org.bson.Document("items", Arrays.asList(new org.bson.Document("itemKey", "i1"), null, new org.bson.Document("itemKey", "i3")));
+
+		Order target = converter.read(Order.class, source);
+
+		assertThat(target.items)
+				.map(it -> it != null ? it.itemKey : null)
+				.containsExactly("i1", null, "i3");
+	}
+
+	@Test // GH-3686
+	void readsArrayContainingNullValue() {
+
+		org.bson.Document source = new org.bson.Document("arrayOfStrings", Arrays.asList("i1", null, "i3"));
+
+		WithArrays target = converter.read(WithArrays.class, source);
+
+		assertThat(target.arrayOfStrings).containsExactly("i1", null, "i3");
+	}
+
+	@Test // GH-3686
+	void readsMapContainingNullValue() {
+
+		org.bson.Document source = new org.bson.Document("mapOfObjects", new org.bson.Document("item1", "i1").append("item2", null).append("item3", "i3"));
+
+		ClassWithMapProperty target = converter.read(ClassWithMapProperty.class, source);
+
+		assertThat(target.mapOfObjects)
+				.containsEntry("item1", "i1")
+				.containsEntry("item2", null)
+				.containsEntry("item3", "i3");
+	}
+
 	static class GenericType<T> {
 		T content;
 	}
@@ -2893,6 +2928,10 @@ static class WithArrayInConstructor {
 
 	}
 
+	static class WithArrays {
+		String[] arrayOfStrings;
+	}
+
 	// DATAMONGO-1898
 
 	// DATACMNS-1278