title: xtext how to obtain root AST element
body: <p>i'm using xtext to build my dsl, and i added a popup action, what i need now is when the action is executed to load the AST  and to convert it, i tried to use the following coe
  to load the root Ast element</p>
  
  <pre><code>/*Environment configuration*/
  StandaloneSetup standalone = new StandaloneSetup();
  standalone.setPlatformUri(workingPath);
  Injector injector = new MyDslStandaloneSetup().createInjectorAndDoEMFRegistration();
  XtextResourceSet resourceSet = injector.getInstance(XtextResourceSet.class);
  resourceSet.addLoadOption(XtextResource.OPTION_RESOLVE_ALL, Boolean.TRUE);
  
  /*File parsing*/
  
  Resource resource = resourceSet.createResource(URI.createURI(uriPrefix + fileName));
  InputStream in = new FileInputStream(workingPath + fileName);
  resource.load(in, resourceSet.getLoadOptions());
  
  if(resource.getContents().size() == 0) {
      System.out.println("The file is empty: no folder has been generated");
      return null;
  }
  Prog model = (Prog) resource.getContents().get(0);
  return model;
  </code></pre>
  
  <p>but don't work and rise this exception :</p>
  
  <pre><code>!MESSAGE Invalid property category path: ValidationPropertiesPage (bundle: org.eclipse.wst.xml.ui, propertyPage: org.eclipse.wst.xml.ui.propertyPage.project.validation)
  org.eclipse.emf.ecore.resource.Resource$IOWrappedException: Content is not allowed in prolog.
      at org.eclipse.emf.ecore.xmi.impl.XMLLoadImpl.load(XMLLoadImpl.java:195)
      at org.eclipse.emf.ecore.xmi.impl.XMLResourceImpl.doLoad(XMLResourceImpl.java:240)
      at org.eclipse.emf.ecore.resource.impl.ResourceImpl.load(ResourceImpl.java:1505)
      at org.xtext.example.mydsl.ui.popup.actions.RunClass.getProg(RunClass.java:203)
      at org.xtext.example.mydsl.ui.popup.actions.RunClass.run(RunClass.java:88)
      at org.eclipse.ui.internal.PluginAction.runWithEvent(PluginAction.java:251)
      at org.eclipse.jface.action.ActionContributionItem.handleWidgetSelection(ActionContributionItem.java:584)
      at org.eclipse.jface.action.ActionContributionItem.access$2(ActionContributionItem.java:501)
      at org.eclipse.jface.action.ActionContributionItem$5.handleEvent(ActionContributionItem.java:411)
      at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
      at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1276)
      at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3554)
      at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3179)
      at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$9.run(PartRenderingEngine.java:1022)
      at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
      at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:916)
      at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:86)
      at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:585)
      at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
      at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:540)
      at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
      at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:124)
      at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
      at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
      at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
      at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:353)
      at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:180)
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke(Method.java:616)
      at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:629)
      at org.eclipse.equinox.launcher.Main.basicRun(Main.java:584)
      at org.eclipse.equinox.launcher.Main.run(Main.java:1438)
      at org.eclipse.equinox.launcher.Main.main(Main.java:1414)
  Caused by: org.xml.sax.SAXParseExceptionpublicId: platform:/resource/prova/a.myDsl; systemId: platform:/resource/prova/a.myDsl; lineNumber: 4; columnNumber: 1; Content is not allowed in prolog.
      at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(ErrorHandlerWrapper.java:198)
      at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.fatalError(ErrorHandlerWrapper.java:177)
      at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:391)
      at com.sun.org.apache.xerces.internal.impl.XMLScanner.reportFatalError(XMLScanner.java:1404)
      at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(XMLDocumentScannerImpl.java:1014)
      at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:625)
      at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:488)
      at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:819)
      at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:748)
      at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:123)
      at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1208)
      at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:525)
      at javax.xml.parsers.SAXParser.parse(SAXParser.java:392)
      at org.eclipse.emf.ecore.xmi.impl.XMLLoadImpl.load(XMLLoadImpl.java:175)
      ... 34 more
  </code></pre>
  
  <p>how  can i load the ASt??</p>