add volatile to the singleton classes

Author: carlosthe19916

src/main/java/io/github/project/openubl/xmlsenderws/webservices/managers/smart/SmartBillServiceConfig.java

@@ -18,7 +18,7 @@
 
 public class SmartBillServiceConfig {
 
-    private static SmartBillServiceConfig instance;
+    private static volatile SmartBillServiceConfig instance;
 
     private String invoiceAndNoteDeliveryURL;
     private String perceptionAndRetentionDeliveryURL;

src/main/java/io/github/project/openubl/xmlsenderws/webservices/providers/ErrorBillServiceRegistry.java

@@ -22,7 +22,7 @@
 
 public class ErrorBillServiceRegistry {
 
-    private static ErrorBillServiceRegistry instance;
+    private static volatile ErrorBillServiceRegistry instance;
 
     private final List<ErrorBillServiceProviderFactory> factories;
 

src/main/java/io/github/project/openubl/xmlsenderws/webservices/utils/SunatErrors.java

@@ -21,7 +21,7 @@
 
 public class SunatErrors {
 
-    private static SunatErrors instance;
+    private static volatile SunatErrors instance;
 
     private final Map<Integer, String> map = new TreeMap<>();