title: ModelCom extension breaks initialization
body: <p>I have a class <code>ModelComExt</code> that extends <code>ModelCom</code>:</p>
  <pre><code>public class ModelComExt extends ModelCom {
      .
      .
      public ModelComExt() {
             super(ValidationReport.reportConformsTrue().getGraph());
      }
      .
      .
      .
      // no overrides or any manipulation with ModelCom's members
  }
  </code></pre>
  <p>When I simply create an instance of <code>ModelComExt</code>, like that:</p>
  <pre><code>public class Main {
      
      public static void main(String[] arguments) {
          JenaSystem.DEBUG_INIT = true;
          new ModelComExt();
      }
         
  }
  </code></pre>
  <p>I get:</p>
  <pre><code>Exception in thread &quot;main&quot; java.lang.ExceptionInInitializerError
      at org.apache.jena.rdf.model.impl.ResourceImpl.fresh(ResourceImpl.java:140)
      at org.apache.jena.rdf.model.impl.ResourceImpl.&lt;init&gt;(ResourceImpl.java:86)
      at org.apache.jena.rdf.model.ResourceFactory$Impl.createResource(ResourceFactory.java:313)
      at org.apache.jena.rdf.model.ResourceFactory.createResource(ResourceFactory.java:92)
      at org.apache.jena.vocabulary.RDFS.resource(RDFS.java:35)
      at org.apache.jena.vocabulary.RDFS$Init.Class(RDFS.java:68)
      at org.apache.jena.vocabulary.RDFS.&lt;clinit&gt;(RDFS.java:40)
      at org.apache.jena.shared.PrefixMapping.&lt;clinit&gt;(PrefixMapping.java:219)
      at com.minimal.failingcase.minimalfailingcase.Main.main(Main.java:17)
  Caused by: java.lang.NullPointerException
      at org.apache.jena.rdf.model.impl.ModelCom.read(ModelCom.java:253)
      at org.apache.jena.util.LocationMapper.initFromPath(LocationMapper.java:179)
      at org.apache.jena.util.LocationMapper.getInternal(LocationMapper.java:69)
      at org.apache.jena.util.FileManagerImpl.makeGlobal(FileManagerImpl.java:142)
      at org.apache.jena.util.FileManagerImpl.get(FileManagerImpl.java:94)
      at org.apache.jena.util.FileManager.getInternal(FileManager.java:34)
      at org.apache.jena.riot.system.IO_JenaReaders.wireIntoJena(IO_JenaReaders.java:41)
      at org.apache.jena.riot.IO_Jena.wireIntoJena(IO_Jena.java:30)
      at org.apache.jena.riot.RIOT.init(RIOT.java:81)
      at org.apache.jena.riot.system.InitRIOT.start(InitRIOT.java:29)
      at org.apache.jena.base.module.Subsystem.lambda$initialize$2(Subsystem.java:117)
      at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
      at org.apache.jena.base.module.Subsystem.forEach(Subsystem.java:193)
      at org.apache.jena.base.module.Subsystem.forEach(Subsystem.java:169)
      at org.apache.jena.base.module.Subsystem.initialize(Subsystem.java:115)
      at org.apache.jena.sys.JenaSystem.init(JenaSystem.java:88)
      at org.apache.jena.graph.NodeFactory.&lt;clinit&gt;(NodeFactory.java:32)
  </code></pre>
  <p>Jena system INIT debug:</p>
  <pre><code>Subsystem.init - start
  Found:
    JenaInitLevel0       [0]
    InitJenaCore         [10]
    InitRIOT             [20]
    InitARQ              [30]
    InitRDFS             [60]
    InitShacl            [95]
  Initialization sequence:
    JenaInitLevel0       [0]
    InitJenaCore         [10]
    InitRIOT             [20]
    InitARQ              [30]
    InitRDFS             [60]
    InitShacl            [95]
  Init: JenaInitLevel0
  Jena initialization
  Init: InitJenaCore
  JenaCore.init - start
  JenaCore.init - finish
  Init: InitRIOT
  RIOT.init - start
  </code></pre>
  <p>Now comes the interesting part - <strong>this only occurs when there is <code>location-mapping.ttl</code> in the directory that gets implicitly loaded</strong>. If there is no <code>location-mapping.ttl</code> in the directory and it doesn't get loaded implicitly, this initialization error does not occur and everything gets initialized successfully when instantiating <code>ModelComExt</code>.</p>
  <p>Edit: Having investigated the nested method calls that occur during <code>ModelComExt</code> instantiation, I have observed that the aforementioned error first occurs when <code>new ModelCom(Factory.createMemGraph())</code> is called. So the following code produces the same error under the same circumstances (<code>location-mapping.ttl</code> is present):</p>
  <pre><code>public class Main {
  
      public static void main(String[] arguments) {
          JenaSystem.DEBUG_INIT = true;
          new ModelCom(Factory.createMemGraph());
      }
  
  }
  </code></pre>