Description of problem:
I create very simple bpmn process using Eclipse Drools plugin. I save it, everything seems ok. I close it. I open it, but now there is this error message:
"An exception occurred while reading in the RuleFlow XML: null See the error log for more details." I have also defined my own WorkItem, and this WorkItem dissapears when reopening bpmn file.

Exception stack trace from workspace/.metadata/.log:

!ENTRY org.drools.eclipse 4 120 2011-11-30 12:37:48.389
!MESSAGE Internal error in Drools Plugin: 
!STACK 0
java.lang.NullPointerException
	at org.jbpm.bpmn2.xml.di.BPMNPlaneHandler.end(BPMNPlaneHandler.java:75)
	at org.drools.xml.ExtensibleXmlParser.endElement(ExtensibleXmlParser.java:414)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(AbstractSAXParser.java:606)
	at com.sun.org.apache.xerces.internal.impl.xs.XMLSchemaValidator.endElement(XMLSchemaValidator.java:854)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEndElement(XMLDocumentFragmentScannerImpl.java:1741)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2898)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:607)
	at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:116)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:488)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:835)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:764)
	at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:123)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1210)
	at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:568)
	at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl.parse(SAXParserImpl.java:302)
	at org.drools.xml.ExtensibleXmlParser.read(ExtensibleXmlParser.java:293)
	at org.drools.xml.ExtensibleXmlParser.read(ExtensibleXmlParser.java:172)
	at org.jbpm.compiler.xml.XmlProcessReader.read(XmlProcessReader.java:46)
	at org.drools.eclipse.flow.bpmn2.editor.BPMNModelEditor.createModel(BPMNModelEditor.java:215)
	at org.drools.eclipse.flow.common.editor.GenericModelEditor.setInput(GenericModelEditor.java:284)
	at org.drools.eclipse.flow.bpmn2.editor.BPMNModelEditor.setInput(BPMNModelEditor.java:105)
	at org.eclipse.gef.ui.parts.GraphicalEditor.init(GraphicalEditor.java:346)
	at org.eclipse.ui.internal.EditorManager.createSite(EditorManager.java:828)
	at org.eclipse.ui.internal.EditorReference.createPartHelper(EditorReference.java:647)
	at org.eclipse.ui.internal.EditorReference.createPart(EditorReference.java:465)
	at org.eclipse.ui.internal.WorkbenchPartReference.getPart(WorkbenchPartReference.java:595)
	at org.eclipse.ui.internal.EditorReference.getEditor(EditorReference.java:289)
	at org.eclipse.ui.internal.WorkbenchPage.busyOpenEditorBatched(WorkbenchPage.java:2945)
	at org.eclipse.ui.internal.WorkbenchPage.busyOpenEditor(WorkbenchPage.java:2850)
	at org.eclipse.ui.internal.WorkbenchPage.access$11(WorkbenchPage.java:2842)
	at org.eclipse.ui.internal.WorkbenchPage$10.run(WorkbenchPage.java:2793)
	at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
	at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:2789)
	at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:2773)
	at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:2764)
	at org.eclipse.ui.ide.IDE.openEditor(IDE.java:651)
	at org.eclipse.ui.ide.IDE.openEditor(IDE.java:610)
	at org.eclipse.jdt.internal.ui.javaeditor.EditorUtility.openInEditor(EditorUtility.java:355)
	at org.eclipse.jdt.internal.ui.javaeditor.EditorUtility.openInEditor(EditorUtility.java:164)
	at org.eclipse.jdt.ui.actions.OpenAction.run(OpenAction.java:249)
	at org.eclipse.jdt.ui.actions.OpenAction.run(OpenAction.java:228)
	at org.eclipse.jdt.ui.actions.SelectionDispatchAction.dispatchRun(SelectionDispatchAction.java:275)
	at org.eclipse.jdt.ui.actions.SelectionDispatchAction.run(SelectionDispatchAction.java:251)
	at org.eclipse.jdt.internal.ui.packageview.PackageExplorerActionGroup.handleOpen(PackageExplorerActionGroup.java:376)
	at org.eclipse.jdt.internal.ui.packageview.PackageExplorerPart$4.open(PackageExplorerPart.java:538)
	at org.eclipse.ui.OpenAndLinkWithEditorHelper$InternalListener.open(OpenAndLinkWithEditorHelper.java:48)
	at org.eclipse.jface.viewers.StructuredViewer$2.run(StructuredViewer.java:866)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
	at org.eclipse.ui.internal.JFaceUtil$1.run(JFaceUtil.java:49)
	at org.eclipse.jface.util.SafeRunnable.run(SafeRunnable.java:175)
	at org.eclipse.jface.viewers.StructuredViewer.fireOpen(StructuredViewer.java:864)
	at org.eclipse.jface.viewers.StructuredViewer.handleOpen(StructuredViewer.java:1152)
	at org.eclipse.jface.viewers.StructuredViewer$6.handleOpen(StructuredViewer.java:1256)
	at org.eclipse.jface.util.OpenStrategy.fireOpenEvent(OpenStrategy.java:275)
	at org.eclipse.jface.util.OpenStrategy.access$2(OpenStrategy.java:269)
	at org.eclipse.jface.util.OpenStrategy$1.handleEvent(OpenStrategy.java:309)
	at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1258)
	at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3588)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3209)
	at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2696)
	at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2660)
	at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2494)
	at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:674)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
	at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:667)
	at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
	at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:123)
	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:344)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
	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:601)
	at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:622)
	at org.eclipse.equinox.launcher.Main.basicRun(Main.java:577)
	at org.eclipse.equinox.launcher.Main.run(Main.java:1410)
	at org.eclipse.equinox.launcher.Main.main(Main.java:1386)


Version-Release number of selected component (if applicable):
Eclipse Indigo 3.7.1
JBoss Tools Development (Drools Eclipse Plug-in 5.2.0.final)

How reproducible:
Easily.

Steps to Reproduce:
1. Start Eclipse with Drools plugin.
2. Create BPMN business process, you can leave it just with start node.
3. Save it.
4. Close it.
5. Open it and error message is shown.
  
Actual results:
Error message occurs when opening business process (.bpmn file) .

Expected results:
No error messages should be visible. If user omits process id by mistake, then he should be cautioned that the process id is necessary before saving and closing the business process in editor.