When adding a "Trigger parameterized build on other projects" or "Trigger/call build on other projects" step in the promotion process in a Job Template, the server raises a NullPointerException when processing the following request:

For Trigger parameterized build on other projects:

http://localhost:8083/job/test-template/descriptorByName/hudson.plugins.parameterizedtrigger.BuildTriggerConfig/checkProjects?value= 

For Trigger/call build on other projects:

http://localhost:8083/job/test-template/descriptorByName/hudson.plugins.parameterizedtrigger.BlockableBuildTriggerConfig/checkProjects?value=

The method checkProjects in the BuildTriggerConfig assume that the @AncestorInPath is not null which is a bug.

Source code here: https://github.com/jenkinsci/parameterized-trigger-plugin/blob/parameterized-trigger-2.29/src/main/java/hudson/plugins/parameterizedtrigger/BuildTriggerConfig.java#L602

How to reproduce:

• Create a Job Template "test-job" at the root
• Tick "Define Promotion Process"
• "Trigger parameterized build on other projects" action
• An ERROR link appears
• "Trigger/call build on other projects" action
• An ERROR link appears

Note: This issue does not block the functionality and this error can be ignored. Specifying an existing project and saving the configuration will make whis error disappears.