test connection to liberty server is ok, but when running the build job, always got below error from the console output.

 
BUILD SUCCESSFUL in 9s
2 actionable tasks: 2 executed
Build step 'Invoke Gradle script' changed build result to SUCCESS
Connecting to IBM WebSphere Liberty Profile...
The following artifacts will be deployed in this order...
-------------------------------------------
JavaHelloWorldApp.war
-------------------------------------------
Stopping Old Application 'JavaHelloWorldApp'...
Error deploying to IBM WebSphere Liberty Profile: org.jenkinsci.plugins.websphere.services.deployment.DeploymentServiceException: Failed to stop artifact: null
at org.jenkinsci.plugins.websphere.services.deployment.LibertyDeploymentService.stopArtifact(LibertyDeploymentService.java:78)
at org.jenkinsci.plugins.websphere_deployer.LibertyDeployerPlugin.stopArtifact(LibertyDeployerPlugin.java:155)
at org.jenkinsci.plugins.websphere_deployer.LibertyDeployerPlugin.perform(LibertyDeployerPlugin.java:97)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:45)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:779)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:720)
at hudson.model.Build$BuildExecution.post2(Build.java:185)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:665)
at hudson.model.Run.execute(Run.java:1766)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:410)