If I create the following Jenkins file in the pipeline editor:

pipeline {
  agent any
  stages {
    stage('Build') {
      steps {
        sh 'mvn clean package'
      }
    }
  }
}

Everything works fine.  If I add the following snippet after the stage line:

	tools {
		maven 'mvn'
		jdk 'jdk8'
	}  

Then the pipeline editor displays a blank page.  If I remove the snipped then all is back normal in the editor.

This Jenkinsfile is the same as the Jenkinsfile in the https://github.com/jenkinsci/jenkins-test-harness-htmlunit/blob/master/Jenkinsfile repository which is utilized by the ci.jenkins.io build site.