Testlink is able to run if no adding action in Iterative Test Build Steps. If I add any kind of action, even just execute shell, the build will failed with below error msg.

 
Started by user zzz
Building in workspace /home/jenkins/jenkins/workspace/test
Preparing TestLink client API.
Using TestLink URL: http://10.0.0.2/testlink/lib/api/xmlrpc/v1/xmlrpc.php

Found 6 automated test cases in TestLink.

Sorting automated test cases by TestLink test plan execution order.

Executing single Build Steps.

Merging build environment variables with data retrieved from TestLink.

[test] $ /bin/bash -xe /tmp/jenkins7699011295981967051.sh
+ rm result.xml
+ echo executing test plan test_plan, part of jenkins_test project
executing test plan test_plan, part of jenkins_test project
+ pwd
/home/jenkins/jenkins/workspace/test
+ cp /home/jenkins/pytest/result.xml ./
+ echo Running test
Running test
Executing iterative Build Steps.

ERROR: Build step failed with exception
java.lang.NullPointerException
at hudson.plugins.testlink.util.TestLinkHelper.addCustomFieldEnvironmentVariableName(TestLinkHelper.java:263)
at hudson.plugins.testlink.util.TestLinkHelper.createTestLinkEnvironmentVariables(TestLinkHelper.java:201)
at hudson.plugins.testlink.util.TestLinkHelper.buildTestCaseEnvVars(TestLinkHelper.java:304)
at hudson.plugins.testlink.TestLinkBuilder.executeIterativeBuildSteps(TestLinkBuilder.java:431)
at hudson.plugins.testlink.TestLinkBuilder.perform(TestLinkBuilder.java:249)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:735)
at hudson.model.Build$BuildExecution.build(Build.java:206)
at hudson.model.Build$BuildExecution.doRun(Build.java:163)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:490)
at hudson.model.Run.execute(Run.java:1735)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:405)
Build step 'Invoke TestLink' marked build as failure
Finished: FAILURE