I'm trying to use the Dynamic Choice Parameter plugin to set up a parameterized build in Jenkins. The parameters should be the names of some directory's in a directory of the working directory of the job (i.e. `/var/lib/jenkins/workspace/JobName/dirToScan/`).

Making the items in the drop down menu can be done in this way:

`def list = ['ls', "/path/to/dirToScan/"].execute().text.tokenize()`

To reach the right directory, I need the name of the job since, as opposed to when a job is executed, the directory in which the this is done is not the `/var/lib/jenkins/workspace/[Job Name]/` directory but the root dir `/`.
The Dynamic Choice Parameter plugin allows the execution of a Groovy script to fill the choice parameter. Using the answer provided here I was able to retrieve the job name. So my final code to build the drop-down menu in the parameterized build menu in Jenkins looks like this:

def build = Thread.currentThread().toString()
def regexp= ".?/job/([^/])/.*"
def match = build =~ regexp
def jobName = match[0][1]

def list = ['ls', "/var/lib/jenkins/workspace/" << jobName << "/dirToScan"].execute().text.tokenize()

Clicking the Build with Parameters link in the Jenkins job opens the page with the drop-down menu (which I named Restore) and also the actual building works great!

But now comes my problem:
If set the job to be build periodically, the build fails with the following console output:

Started by timer
Building in workspace /var/lib/jenkins/workspace/JobName
FATAL: Null value not allowed as an environment variable: Restore
java.lang.IllegalArgumentException: Null value not allowed as an environment variable: Restore
at hudson.EnvVars.put(EnvVars.java:356)
at hudson.model.StringParameterValue.buildEnvironment(StringParameterValue.java:56)
at hudson.model.ParametersAction.buildEnvVars(ParametersAction.java:88)
at hudson.model.AbstractBuild.getEnvironment(AbstractBuild.java:929)
at hudson.plugins.mercurial.MercurialSCM.checkout(MercurialSCM.java:533)
at hudson.scm.SCM.checkout(SCM.java:484)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1270)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:609)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:531)
at hudson.model.Run.execute(Run.java:1750)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:89)
at hudson.model.Executor.run(Executor.java:240)
Finished: FAILURE

This does not happen if I just populate the drop-down menu in a 'normal' way, e.g.: def list = ["Option 1", "Option 2", "Option 3"]

This might be a bug in the plugin, or Jenkins being unable to execute the Groovy code if it starts a job by itself. The problem only occurs with a periodic build, not if I start the build myself and it it independent of the chosen option in the drop-down menu.