Spring boot - Null pointer exception while mocking MongoTemplate.getCollection() - mongodb

Question

I have written a service class which uses MongoTemplate.getCollection().
But while writing the test case I am mockingMongoTempalte and MongoCollection but getting NullPointerException.
Following is the code.
Service class
import com.mongodb.client.MongoCollection;
import org.bson.Document;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.mongodb.core.MongoTemplate;
import org.springframework.stereotype.Service;
import java.util.ArrayList;
import java.util.NoSuchElementException;
#Service
public class ABCService {
#Autowired
MongoTemplate mongoTemplate;
public byte[] exportFrom(String collectionName) {
if(!mongoTemplate.collectionExists(collectionName))
throw new NoSuchElementException("Collection does not exist");
MongoCollection<Document> collection = mongoTemplate.getCollection(collectionName);
do something
} catch (Exception e) {
throw new RuntimeException(e);
}
}
}
Test Class:
import com.mongodb.BasicDBObject;
import com.mongodb.client.MongoCollection;
import org.bson.Document;
import org.json.JSONObject;
import org.junit.Before;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import org.springframework.data.mongodb.core.MongoTemplate;
import static org.mockito.Mockito.*;
#ExtendWith(MockitoExtension.class)
class ABCServiceTest {
#Mock
private MongoTemplate mongoTemplate;
#Mock
private MongoCollection<Document> documentMongoDBCollection;
#InjectMocks
private ABCService abcService ;
#Test
void exportFrom() throws Exception {
JSONObject obj = new JSONObject();
obj.put("_id", "1234");
obj.put("accessField", "xyz 123");
Document doc = Document.parse(obj.toString());
byte[] pos = obj.toString().getBytes();
when(mongoTemplate.collectionExists("DataCollection"))
.thenReturn(true);
when(mongoTemplate.getCollection(any(String.class)))
.thenReturn(documentMongoDBCollection); // **This returning Null pointer exception**
when(abcService .exportFrom("DataCollection"))
.thenReturn(pos);
byte[] result = abcService .exportFrom("DataCollection");
Assertions.assertNotNull(result);
verify(mongoTemplate,times(1)).collectionExists("DataCollection");
verify(mongoTemplate,times(1)).getCollection("DataCollection");
}
}
Expecting to throw object to complete the mocking of Mongo template.