Doing logout using backChannelLogout its giving null

Hi All,
When I am clearing the session by using below code
for (UserSessionModel userSession : userSessions) {

        AuthenticationManager.backchannelLogout(context.getSession(), userSession, false);

    }

I am getting following error.Please help me out
[org.keycloak.services] (default task-3) KC-SERVICES0013: Failed authentication: java.lang.NullPointerException
at org.keycloak.keycloak-services@9.0.3//org.keycloak.authentication.DefaultAuthenticationFlow.processResult(DefaultAuthenticationFlow.java:477)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.authentication.DefaultAuthenticationFlow.processSingleFlowExecutionModel(DefaultAuthenticationFlow.java:440)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.authentication.DefaultAuthenticationFlow.processFlow(DefaultAuthenticationFlow.java:247)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.authentication.DefaultAuthenticationFlow.processSingleFlowExecutionModel(DefaultAuthenticationFlow.java:381)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.authentication.DefaultAuthenticationFlow.processFlow(DefaultAuthenticationFlow.java:247)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.authentication.DefaultAuthenticationFlow.processSingleFlowExecutionModel(DefaultAuthenticationFlow.java:381)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.authentication.DefaultAuthenticationFlow.processFlow(DefaultAuthenticationFlow.java:247)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.authentication.AuthenticationProcessor.authenticateOnly(AuthenticationProcessor.java:976)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.authentication.AuthenticationProcessor.authenticate(AuthenticationProcessor.java:838)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.protocol.AuthorizationEndpointBase.handleBrowserAuthenticationRequest(AuthorizationEndpointBase.java:150)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint.buildAuthorizationCodeAuthorizationResponse(AuthorizationEndpoint.java:464)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint.process(AuthorizationEndpoint.java:160)
at org.keycloak.keycloak-services@9.0.3//org.keycloak.protocol.oidc.endpoints.AuthorizationEndpoint.buildGet(AuthorizationEndpoint.java:111)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)