title: GoCD - Downstream Pipelines are scheduled and asgined but not starting
body: <p>I´m running the GO-CD Server and Agents as Alpine Docker containers  on a linux machine</p>
  
  <p>I´m adding several pipelines through the json config plugin. The Server seems to start the first Pipeline and upload all the materials. Then he triggers 2 downstream pipes who´ll fetch the materials. </p>
  
  <p>The 2 Downstream pipelines are beeing scheduled and asingned to an agent but they are not starting.</p>
  
  <p>I´ve stopped the stages to see if the error was still comming and they stopped too. </p>
  
  <pre><code>2018-08-29 15:34:28,029 ERROR [85@MessageListener for WorkFinder] JMSMessageListenerAdapter:77 - Exception thrown in message handling by listener com.thoughtworks.go.server.messaging.scheduling.WorkFinder@2c731a16
  java.lang.NullPointerException: null
  2018-08-29 15:34:36,919 ERROR [85@MessageListener for WorkFinder] JMSMessageListenerAdapter:77 - Exception thrown in message handling by listener com.thoughtworks.go.server.messaging.scheduling.WorkFinder@2c731a16
  java.lang.NullPointerException: null
  2018-08-29 15:34:38,040 ERROR [85@MessageListener for WorkFinder] JMSMessageListenerAdapter:77 - Exception thrown in message handling by listener com.thoughtworks.go.server.messaging.scheduling.WorkFinder@2c731a16
  java.lang.NullPointerException: null
  </code></pre>
  
  <p>Thats how the some of the pipelines are configured in the repository</p>
  
  <pre><code>{
    "format_version" : 3,
    "group": "group1",
    "name": "Test-Pipeline-Stage",
    "label_template": "${COUNT}",
    "enable_pipeline_locking" : false,
    "parameters": [],
    "materials": [
      {
        "type": "git",
        "url": ""
        }
      ],
    "stages": [
      {
        "name": "Test",
        "fetch_materials": true,
        "never_cleanup_artifacts": false,
        "clean_working_directory": false,
        "approval" : null,
        "jobs": [{
          "name": "RunUnittestAndCoverage",
          "run_instance_count" : "all",
          "environment_variables": [],
          "timeout": 1,
          "tabs": [
             {
               "name": "coverage",
               "path": "coverage/coverage.html/index.html"
             }
           ],
          "artifacts": [
            {
              "source":"project.tar.gz",
              "destination":"project_source",
              "type":"build"
            },
            {
              "source": "test_result.xml",
              "destination": "testoutput",
              "type": "test"
            },
            {
              "source": "coverage.html",
              "destination": "coverage",
              "type": "build"
            },
            {
              "source": "coverage.xml",
              "destination": "coverage",
              "type": "build"
            }
            ],
          "tasks":[
            {
              "type": "exec",
              "run_if": "passed",
              "on_cancel": null,
              "command":"tox"
            },
            {
             "type":"exec",
              "run_if": "passed",
              "on_cancel": null,
              "command":"/usr/bin/xmllint",
              "arguments": [
                  "--format",
                  "test_resulto.xml",
                  "--output",
                  "test_result.xml"
                ]
              },
            {
              "type":"exec",
              "run_if": "passed",
              "on_cancel": null,
              "command": "/bin/ls",
              "arguments":[
                "-al"
              ]
            },
            {
              "type":"exec",
              "run_if": "passed",
              "on_cancel": null,
              "command": "/bin/tar",
              "arguments":[
                "-cvzf",
                "project.tar.gz",
                "/godata/pipelines/Test-Pipeline-Stage"
              ]
            }
            ]
          }
        ]
      }
    ]
  }
  </code></pre>
  
  <hr>
  
  <pre><code>{
    "format_version" : 3,
    "group": "group1",
    "name": "Docker-Deployment-Pipeline-Stage",
    "label_template": "${COUNT}",
    "enable_pipeline_locking" : false,
    "parameters": [],
    "materials": [
        {
          "type": "dependency",
          "pipeline": "Build-Pipeline-Stage",
          "stage": "BuildPythonProduct",
          "name": "project_source"
        },
        {
          "type": "dependency",
          "pipeline": "Analyse-Pipeline-Stage",
          "stage":"Analyse",
          "name":"analyse_result"
        }
      ],
    "stages":[
      {
        "name": "BuildDockerImage",
        "fetch_materials": true,
        "never_cleanup_artifacts": false,
        "clean_working_directory": false,
        "approval" : null,
        "jobs":[
          {
            "name": "BuildImage",
            "run_instance_count" : "all",
            "environment_variables": [],
            "timeout": 1,
            "tasks": [
              {
                  "type": "fetch",
                  "artifact_origin": "gocd",
                  "run_if": "any",
                  "pipeline": "Build-Pipeline-Stage",
                  "stage": "BuildPythonProduct",
                  "job": "BuildAndUpLoadWheel",
                  "is_source_a_file": true,
                  "source": "docker/Dockerfile",
                  "destination": ""
              },
              {
                  "type": "fetch",
                  "artifact_origin": "gocd",
                  "run_if": "any",
                  "pipeline": "Build-Pipeline-Stage",
                  "stage": "BuildPythonProduct",
                  "job": "BuildAndUpLoadWheel",
                  "is_source_a_file": true,
                  "source": "docker/production.ini",
                  "destination": ""
              },
              {
                  "type":"exec",
                  "run_if": "passed",
                  "on_cancel": null,
                  "command":"/usr/bin/docker",
                  "arguments": [
                   "build",
                   "-t",
                   "project_creator",
                   "."
                ]
              }
            ]
          }
        ]
      }
    ]
  }
  </code></pre>