Here's another exception (using 12/12/12 nightly)

org.eclipse.core.runtime.AssertionFailedException: null argument:
at org.eclipse.core.runtime.Assert.isNotNull(Assert.java:85)
at org.eclipse.core.runtime.Assert.isNotNull(Assert.java:73)
at org.eclipse.jdt.ui.text.folding.DefaultJavaFoldingStructureProvider.computeCurrentStructure(DefaultJavaFoldingStructureProvider.java:1388)
at org.eclipse.jdt.ui.text.folding.DefaultJavaFoldingStructureProvider.update(DefaultJavaFoldingStructureProvider.java:886)
at org.eclipse.jdt.ui.text.folding.DefaultJavaFoldingStructureProvider.access$9(DefaultJavaFoldingStructureProvider.java:876)
at org.eclipse.jdt.ui.text.folding.DefaultJavaFoldingStructureProvider$ElementChangedListener.elementChanged(DefaultJavaFoldingStructureProvider.java:361)
at org.eclipse.jdt.internal.core.DeltaProcessor$4.run(DeltaProcessor.java:1670)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.jdt.internal.core.DeltaProcessor.notifyListeners(DeltaProcessor.java:1660)
at org.eclipse.jdt.internal.core.DeltaProcessor.fireReconcileDelta(DeltaProcessor.java:1512)
at org.eclipse.jdt.internal.core.DeltaProcessor.fire(DeltaProcessor.java:1471)
at org.eclipse.jdt.internal.core.JavaModelOperation.run(JavaModelOperation.java:769)
at org.eclipse.jdt.internal.core.JavaModelOperation.runOperation(JavaModelOperation.java:788)
at org.eclipse.jdt.internal.core.CompilationUnit.reconcile(CompilationUnit.java:1244)
at scala.tools.eclipse.javaelements.ScalaSourceFile.reconcile(ScalaSourceFile.scala:83)
at org.eclipse.jdt.internal.ui.text.java.JavaReconcilingStrategy.reconcile(JavaReconcilingStrategy.java:126)
at org.eclipse.jdt.internal.ui.text.java.JavaReconcilingStrategy.access$0(JavaReconcilingStrategy.java:108)
at org.eclipse.jdt.internal.ui.text.java.JavaReconcilingStrategy$1.run(JavaReconcilingStrategy.java:89)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.jdt.internal.ui.text.java.JavaReconcilingStrategy.reconcile(JavaReconcilingStrategy.java:87)
at org.eclipse.jdt.internal.ui.text.java.JavaReconcilingStrategy.reconcile(JavaReconcilingStrategy.java:151)
at org.eclipse.jdt.internal.ui.text.CompositeReconcilingStrategy.reconcile(CompositeReconcilingStrategy.java:86)
at org.eclipse.jdt.internal.ui.text.JavaCompositeReconcilingStrategy.reconcile(JavaCompositeReconcilingStrategy.java:104)
at org.eclipse.jface.text.reconciler.MonoReconciler.process(MonoReconciler.java:77)
at org.eclipse.jface.text.reconciler.AbstractReconciler$BackgroundThread.run(AbstractReconciler.java:206)