.NET framework error at mdlModelRef_getRootParent(DgnModelRef* )

Using Geopak SS4 .878, this is the error stack:

************** Exception Text **************
System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
   at mdlModelRef_getRootParent(DgnModelRef* )
   at Bentley.Civil.Model.MSPlugIn.DgnModelRefPathHolder.GetForCache(DgnModelRef* referenceModel, DgnCache* dgnCache)
   at Bentley.Civil.Model.MSPlugIn.MSDgnFileInfo.GetUniversalModelReferenceInternal(DgnRepositoryConnection dgnConnection, DgnModelRef* referenceModel, DgnCache* dgnCache)
   at Bentley.Civil.Model.MSPlugIn.MSDgnFileInfo.GetUniversalModelReference(DgnRepositoryConnection dgnConnection, DgnModelRef* referenceModel, DgnCache* dgnCache)
   at Bentley.Civil.Model.MSPlugIn.ModelPersistenceStrategy.GetInstanceIdFromElement(IElementRef* elRef, DgnModelRef* candidateTargetModel)
   at Bentley.Civil.Model.MSPlugIn.ModelPersistenceStrategy.GetObjectKey(DgnModelRef* model, IElementRef* elRef)
   at Bentley.Civil.Model.MSPlugIn.MstnPresentationEnabler.GetObjectKey(DgnModelRef* model, IElementRef* elRef)
   at Bentley.Civil.CadSystem.Microstation.MstnPresentationEnablersManager.GetObjectKeyInternal(ElemHandle* elemHandle, Boolean getParentOfComponent, Boolean ecKeyOnly)
   at Bentley.Civil.CadSystem.Microstation.MstnPresentationEnablersManager.GetObjectKey(Element element)
   at Bentley.Civil.CadSystem.Microstation.MstnObjectPropertyEnabler.FilterProperty(String requestor, String controlType, String propertyEnabler, String category, String stateFileName, IECPropertyValue propertyValue, ECInstanceListSet instanceListSet)
   at Bentley.ECObjects.UI.ECMultiInstancePropertyAdapter.FilterProperty(IECPropertyValue propertyValue, String categoryName)
   at Bentley.ECObjects.UI.ECMultiInstancePropertyAdapter.AddMemberProperties(IECValueContainer valueContainer, ECInstanceListSet objectSet, Int32 instanceNumber)
   at Bentley.ECObjects.UI.ECMultiInstancePropertyAdapter.AddPropertiesInInstanceList(ECInstanceList instanceList, ECInstanceListSet objectSet, Boolean differingInstanceLists)
   at Bentley.ECObjects.UI.ECMultiInstancePropertyAdapter.GetTypeDescriptors()
   at Bentley.ECObjects.UI.ECPropertyPane.ShowInstanceLists()
   at Bentley.ECObjects.UI.ECPropertyPane.SetInstanceListSet(ECInstanceListSet instanceListSetLeft, ECInstanceListSet instanceListSetRight)
   at Bentley.ECObjects.UI.ECPropertyPane.SetInstanceListSet(ECInstanceListSet instanceListSet)
   at Bentley.DesignExplorer.NavigatorPropertyGrid.Reload()
   at Bentley.DesignExplorer.NavigatorPropertyGrid.OnReloadTimeout(Object senderIn, EventArgs eventArgsIn)
   at System.Windows.Forms.Timer.OnTick(EventArgs e)
   at System.Windows.Forms.Timer.TimerNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

I have a series of files:

1. Primary points and breaks
2. Base terrain model, which attaches the primary points and breaks and uses a graphic filter to create the model.
3. Improved terrain model, which attaches the base terrain model and merges a few segments of additional survey data.
4. Working terrain model, which clips the improved terrain model to something much smaller.

My most recent work involved these steps:

1. In file 1, I made some edits to points and breaks.
2. In file 2, I refreshed the base terrain model from source (the graphic filter). It looked fine.
3. I opened file 3 to see if it had been updated. It didn't really look like it, at first, so I attempted to attach file 1 to confirm that the edits were taken care of. That's when this error was raised.