Example (using openconfig-network-instance@2015-10-18):

<rpc message-id="m-2" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<get-config>
<source>
<running/>
</source>
<filter xmlns:ns0="urn:ietf:params:xml:ns:netconf:base:1.0" ns0:type="subtree">
<network-instances xmlns="http://openconfig.net/yang/network-instance">
<network-instance>
<name>global-bgp</name>
</network-instance>
</network-instances>
</filter>
</get-config>
</rpc>]]>]]>

Stack trace:

java.lang.NullPointerException: null
    at org.opendaylight.netconf.mdsal.connector.ops.get.FilterContentValidator.getKeyValues(FilterContentValidator.java:221) ~[mdsal-netconf-connector-1.3.1.jar:na]
    at org.opendaylight.netconf.mdsal.connector.ops.get.FilterContentValidator.appendKeyIfPresent(FilterContentValidator.java:177) ~[mdsal-netconf-connector-1.3.1.jar:na]
    at org.opendaylight.netconf.mdsal.connector.ops.get.FilterContentValidator.getFilterDataRoot(FilterContentValidator.java:163) ~[mdsal-netconf-connector-1.3.1.jar:na]
    at org.opendaylight.netconf.mdsal.connector.ops.get.FilterContentValidator.validate(FilterContentValidator.java:72) ~[mdsal-netconf-connector-1.3.1.jar:na]
    at org.opendaylight.netconf.mdsal.connector.ops.get.AbstractGet.getInstanceIdentifierFromFilter(AbstractGet.java:150) ~[mdsal-netconf-connector-1.3.1.jar:na]
    at org.opendaylight.netconf.mdsal.connector.ops.get.AbstractGet.getDataRootFromFilter(AbstractGet.java:134) ~[mdsal-netconf-connector-1.3.1.jar:na]
    at org.opendaylight.netconf.mdsal.connector.ops.get.GetConfig.handleWithNoSubsequentOperations(GetConfig.java:58) ~[mdsal-netconf-connector-1.3.1.jar:na]
    at org.opendaylight.netconf.util.mapping.AbstractSingletonNetconfOperation.handle(AbstractSingletonNetconfOperation.java:26) ~[netconf-util-1.3.1.jar:na]
    at org.opendaylight.netconf.util.mapping.AbstractNetconfOperation.handle(AbstractNetconfOperation.java:104) ~[netconf-util-1.3.1.jar:na]
    at org.opendaylight.netconf.impl.osgi.NetconfOperationRouterImpl$NetconfOperationExecution.execute(NetconfOperationRouterImpl.java:174) ~[netconf-impl-1.3.1.jar:na]
    at org.opendaylight.netconf.impl.osgi.NetconfOperationRouterImpl.executeOperationWithHighestPriority(NetconfOperationRouterImpl.java:115) ~[netconf-impl-1.3.1.jar:na]
    at org.opendaylight.netconf.impl.osgi.NetconfOperationRouterImpl.onNetconfMessage(NetconfOperationRouterImpl.java:86) ~[netconf-impl-1.3.1.jar:na]
    at org.opendaylight.netconf.impl.NetconfServerSessionListener.processDocument(NetconfServerSessionListener.java:124) [netconf-impl-1.3.1.jar:na]
    at org.opendaylight.netconf.impl.NetconfServerSessionListener.onMessage(NetconfServerSessionListener.java:89) [netconf-impl-1.3.1.jar:na]
    at org.opendaylight.netconf.impl.NetconfServerSessionListener.onMessage(NetconfServerSessionListener.java:32) [netconf-impl-1.3.1.jar:na]
    at org.opendaylight.netconf.nettyutil.AbstractNetconfSession.handleMessage(AbstractNetconfSession.java:65) [netconf-netty-util-1.3.1.jar:na]
    at org.opendaylight.netconf.nettyutil.AbstractNetconfSession.handleMessage(AbstractNetconfSession.java:35) [netconf-netty-util-1.3.1.jar:na]
    at org.opendaylight.protocol.framework.AbstractProtocolSession.channelRead0(AbstractProtocolSession.java:53) [protocol-framework-1.0.1.jar:na]
    at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105) [netty-transport-4.1.8.Final.jar:4.1.8.Final]