The MonitoringSchema should have a default constructor

The MonitoringSchema should have a default constructor like the MonitoringSession, otherwise, it will fail while execute the following code in JaxBSerializer
JAXB_CONTEXT = JAXBContext.newInstance(NetconfState.class);

it will result in
java.lang.NoClassDefFoundError: Could not initialize class org.opendaylight.netconf.monitoring.xml.JaxBSerializer
at org.opendaylight.netconf.monitoring.Get.handle(Get.java:67)
at org.opendaylight.netconf.impl.osgi.NetconfOperationRouterImpl$NetconfOperationExecution.execute(NetconfOperationRouterImpl.java:182)
at org.opendaylight.netconf.notifications.impl.ops.Get.handle(Get.java:56)
at org.opendaylight.netconf.impl.osgi.NetconfOperationRouterImpl$NetconfOperationExecution.execute(NetconfOperationRouterImpl.java:182)
at org.opendaylight.netconf.impl.osgi.NetconfOperationRouterImpl.executeOperationWithHighestPriority(NetconfOperationRouterImpl.java:111)
at org.opendaylight.netconf.impl.osgi.NetconfOperationRouterImpl.onNetconfMessage(NetconfOperationRouterImpl.java:84)
at org.opendaylight.netconf.impl.NetconfServerSessionListener.processDocument(NetconfServerSessionListener.java:113)
at org.opendaylight.netconf.impl.NetconfServerSessionListener.onMessage(NetconfServerSessionListener.java:86)
at org.opendaylight.netconf.impl.NetconfServerSessionListener.onMessage(NetconfServerSessionListener.java:29)