When one uses Predefined Generator Parameters to trigger a parameterized build, this exception is raised where VARIABLE_NAME is the name of a defined Generator Parameter:

java.lang.IllegalArgumentException: Null value not allowed as an environment variable: VARIABLE_NAME
at hudson.EnvVars.put(EnvVars.java:356)
at hudson.model.StringParameterValue.buildEnvironment(StringParameterValue.java:56)
at hudson.model.ParametersAction.buildEnvVars(ParametersAction.java:85)
at hudson.model.AbstractBuild.getEnvironment(AbstractBuild.java:883)
at hudson.plugins.parameterizedtrigger.AbstractBuildParameters.getEnvironment(AbstractBuildParameters.java:47)
at org.jenkinsci.plugins.jobgenerator.parameterizedtrigger.PredefinedGeneratorParameters.getAction(PredefinedGeneratorParameters.java:59)
at org.jenkinsci.plugins.jobgenerator.GeneratorRun$GeneratorImpl.gatherDownstreamGenerators(GeneratorRun.java:493)
at org.jenkinsci.plugins.jobgenerator.GeneratorRun$GeneratorImpl.doRun(GeneratorRun.java:299)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:523)
at hudson.model.Run.execute(Run.java:1688)
at org.jenkinsci.plugins.jobgenerator.GeneratorRun.run(GeneratorRun.java:284)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:231)
at hudson.model.OneOffExecutor.run(OneOffExecutor.java:43)