error handling improved when source xml for drugs is invalid

annotation/src/main/java/lcsb/mapviewer/annotation/services/ChEMBLParser.java

@@ -496,10 +496,18 @@ public class ChEMBLParser extends DrugAnnotation implements IExternalService {
 			Document document = getXmlDocumentFromString(content);
 
 			Node response = getNode("response", document);
+			if (response == null) {
+				throw new DrugSearchException("Invalid source XML for drug: " + drugId);
+			}
 			Node moleculesNode = getNode("molecules", response);
+			if (moleculesNode == null) {
+				throw new DrugSearchException("Invalid source XML for drug: " + drugId);
+			}
 			Node moleculeNode = getNode("molecule", moleculesNode);
+			if (moleculeNode == null) {
+				throw new DrugSearchException("Invalid source XML for drug: " + drugId);
+			}
 			return parseDrug(moleculeNode);
-
 		} catch (IOException e) {
 			throw new DrugSearchException("Problem with accessing external database for id: " + drugId, e);
 		} catch (InvalidXmlSchemaException e) {