I have created a template with the following Trigger

Script - After Build
Trigger Script: build.result.toString().equals('SUCCESS') && build.getBuildVariables().get("publish").equals('true');

Then I assign it to the project, that is properly configured, it has a 'publish' variable that was checked in the build, and when I built the project I got this error.

Important Detail: With the exactly same configuration but without using templates, it work flawlessly.

BUILD SUCCESSFUL
Total time: 2 minutes 38 seconds
ERROR: Build step failed with exception
java.lang.NullPointerException
at hudson.plugins.emailext.plugins.trigger.AbstractScriptTrigger.createEngine(AbstractScriptTrigger.java:81)
at hudson.plugins.emailext.plugins.trigger.AbstractScriptTrigger.trigger(AbstractScriptTrigger.java:56)
at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:274)
at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:258)
at org.jenkinsci.plugins.emailext_template.ExtendedEmailTemplatePublisher.perform(ExtendedEmailTemplatePublisher.java:67)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:782)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:723)
at hudson.model.Build$BuildExecution.post2(Build.java:185)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:668)
at hudson.model.Run.execute(Run.java:1763)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:410)
Build step 'Editable Email Notification Templates' marked build as failure
Finished: FAILURE