If I open a file outside of a project with “Open file…” I get this:

java.lang.NullPointerException
	at org.metaborg.spoofax.eclipse.resource.EclipseResourceService.unresolve(EclipseResourceService.java:88)
	at org.metaborg.spoofax.eclipse.editor.SpoofaxEditor.createSourceViewer(SpoofaxEditor.java:263)
	at org.eclipse.ui.texteditor.AbstractTextEditor.createPartControl(AbstractTextEditor.java:3423)
	at org.eclipse.ui.texteditor.StatusTextEditor.createPartControl(StatusTextEditor.java:54)
	at org.eclipse.ui.texteditor.AbstractDecoratedTextEditor.createPartControl(AbstractDecoratedTextEditor.java:447)
	at org.eclipse.ui.internal.e4.compatibility.CompatibilityPart.createPartControl(CompatibilityPart.java:141)
	at org.eclipse.ui.internal.e4.compatibility.CompatibilityEditor.createPartControl(CompatibilityEditor.java:99)
	at org.eclipse.ui.internal.e4.compatibility.CompatibilityPart.create(CompatibilityPart.java:327)
	at sun.reflect.GeneratedMethodAccessor61.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.eclipse.e4.core.internal.di.MethodRequestor.execute(MethodRequestor.java:55)
	at org.eclipse.e4.core.internal.di.InjectorImpl.processAnnotated(InjectorImpl.java:888)
	at org.eclipse.e4.core.internal.di.InjectorImpl.processAnnotated(InjectorImpl.java:869)
	at org.eclipse.e4.core.internal.di.InjectorImpl.inject(InjectorImpl.java:120)
	at org.eclipse.e4.core.internal.di.InjectorImpl.internalMake(InjectorImpl.java:337)
	at org.eclipse.e4.core.internal.di.InjectorImpl.make(InjectorImpl.java:258)
	at org.eclipse.e4.core.contexts.ContextInjectionFactory.make(ContextInjectionFactory.java:162)
	at org.eclipse.e4.ui.internal.workbench.ReflectionContributionFactory.createFromBundle(ReflectionContributionFactory.java:104)
	at org.eclipse.e4.ui.internal.workbench.ReflectionContributionFactory.doCreate(ReflectionContributionFactory.java:73)
	at org.eclipse.e4.ui.internal.workbench.ReflectionContributionFactory.create(ReflectionContributionFactory.java:55)
	at org.eclipse.e4.ui.workbench.renderers.swt.ContributedPartRenderer.createWidget(ContributedPartRenderer.java:127)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.createWidget(PartRenderingEngine.java:983)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.safeCreateGui(PartRenderingEngine.java:662)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.safeCreateGui(PartRenderingEngine.java:766)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.access$2(PartRenderingEngine.java:737)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$7.run(PartRenderingEngine.java:731)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.createGui(PartRenderingEngine.java:715)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl$1.handleEvent(PartServiceImpl.java:95)
	at org.eclipse.e4.ui.services.internal.events.UIEventHandler$1.run(UIEventHandler.java:40)
	at org.eclipse.swt.widgets.Synchronizer.syncExec(Synchronizer.java:187)
	at org.eclipse.ui.internal.UISynchronizer.syncExec(UISynchronizer.java:156)
	at org.eclipse.swt.widgets.Display.syncExec(Display.java:4590)
	at org.eclipse.e4.ui.internal.workbench.swt.E4Application$1.syncExec(E4Application.java:218)
	at org.eclipse.e4.ui.services.internal.events.UIEventHandler.handleEvent(UIEventHandler.java:36)
	at org.eclipse.equinox.internal.event.EventHandlerWrapper.handleEvent(EventHandlerWrapper.java:197)
	at org.eclipse.equinox.internal.event.EventHandlerTracker.dispatchEvent(EventHandlerTracker.java:197)
	at org.eclipse.equinox.internal.event.EventHandlerTracker.dispatchEvent(EventHandlerTracker.java:1)
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:230)
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:148)
	at org.eclipse.equinox.internal.event.EventAdminImpl.dispatchEvent(EventAdminImpl.java:135)
	at org.eclipse.equinox.internal.event.EventAdminImpl.sendEvent(EventAdminImpl.java:78)
	at org.eclipse.equinox.internal.event.EventComponent.sendEvent(EventComponent.java:39)
	at org.eclipse.e4.ui.services.internal.events.EventBroker.send(EventBroker.java:81)
	at org.eclipse.e4.ui.internal.workbench.UIEventPublisher.notifyChanged(UIEventPublisher.java:59)
	at org.eclipse.emf.common.notify.impl.BasicNotifierImpl.eNotify(BasicNotifierImpl.java:374)
	at org.eclipse.e4.ui.model.application.ui.impl.ElementContainerImpl.setSelectedElement(ElementContainerImpl.java:171)
	at org.eclipse.e4.ui.internal.workbench.ModelServiceImpl.showElementInWindow(ModelServiceImpl.java:488)
	at org.eclipse.e4.ui.internal.workbench.ModelServiceImpl.bringToTop(ModelServiceImpl.java:454)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl.delegateBringToTop(PartServiceImpl.java:705)
	at org.eclipse.e4.ui.internal.workbench.PartServiceImpl.showPart(PartServiceImpl.java:1142)
	at org.eclipse.ui.internal.WorkbenchPage.busyOpenEditor(WorkbenchPage.java:3210)
	at org.eclipse.ui.internal.WorkbenchPage.access$23(WorkbenchPage.java:3125)
	at org.eclipse.ui.internal.WorkbenchPage$9.run(WorkbenchPage.java:3107)
	at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
	at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:3102)
	at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:3066)
	at org.eclipse.ui.internal.WorkbenchPage.openEditor(WorkbenchPage.java:3047)
	at org.eclipse.ui.ide.IDE.openEditorOnFileStore(IDE.java:1139)
	at org.eclipse.ui.internal.ide.actions.OpenLocalFileAction.run(OpenLocalFileAction.java:107)
	at org.eclipse.ui.internal.ide.actions.OpenLocalFileAction.run(OpenLocalFileAction.java:76)
	at org.eclipse.ui.internal.PluginAction.runWithEvent(PluginAction.java:253)
	at org.eclipse.ui.internal.WWinPluginAction.runWithEvent(WWinPluginAction.java:237)
	at org.eclipse.jface.action.ActionContributionItem.handleWidgetSelection(ActionContributionItem.java:595)
	at org.eclipse.jface.action.ActionContributionItem.access$2(ActionContributionItem.java:511)
	at org.eclipse.jface.action.ActionContributionItem$5.handleEvent(ActionContributionItem.java:420)
	at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
	at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4454)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1388)
	at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3799)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3409)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$9.run(PartRenderingEngine.java:1151)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:1032)
	at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:148)
	at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:636)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
	at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:579)
	at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:150)
	at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:135)
	at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:134)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:380)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:235)
	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:606)
	at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:648)
	at org.eclipse.equinox.launcher.Main.basicRun(Main.java:603)
	at org.eclipse.equinox.launcher.Main.run(Main.java:1465)
	at org.eclipse.equinox.launcher.Main.main(Main.java:1438)

Eclipse: org.eclipse.epp.package.standard.feature.feature.group 4.4.2.20150219-0708
Spoofax: org.metaborg.spoofax.eclipse 1.5.0.20150824-224106-new-spoofax-plugin
System: Linux amd64 3.13.0-37-generic

Submitted by Jeff Smits on 25 August 2015 at 23:14

Fixed. As a bonus it is now also possible to do reference resolution to files outside of the Eclipse workspace.