NullReferenceException in Document.get_Characters

Similar steps to repro as seen here.

The user used a "Close All Documents" operation in our application. He indicates that when the last open document was about to close, this exception occurred:

System.NullReferenceException: Object reference not set to an instance of an object.
at ActiproSoftware.SyntaxEditor.Document.get_Characters(Int32 index)
at ActiproSoftware.SyntaxEditor.DisplayLineCollection.a(Int32 A_0)
at ActiproSoftware.SyntaxEditor.DisplayLine.get_Length()
at ActiproSoftware.SyntaxEditor.DisplayLine.get_EndOffset()
at ActiproSoftware.SyntaxEditor.DisplayLineCollection.e(Int32 A_0)
at ActiproSoftware.SyntaxEditor.EditorView.a(Boolean A_0, Boolean A_1)
at ActiproSoftware.SyntaxEditor.EditorView.ArrangeCore()
at ActiproSoftware.WinUICore.UIElement.Arrange(Rectangle newBounds)
at ActiproSoftware.SyntaxEditor.SyntaxEditor.ArrangeCore()
at ActiproSoftware.WinUICore.UIControl.Arrange(Rectangle newBounds)
at ActiproSoftware.WinUICore.UIControl.UpdateLayout()
at ActiproSoftware.WinUICore.UIControl.Invalidate(InvalidationLevels levels, InvalidationTypes types)
at ActiproSoftware.WinUICore.UIControl.OnLayout(LayoutEventArgs e)
at System.Windows.Forms.Control.PerformLayout(LayoutEventArgs args)
at System.Windows.Forms.Control.PerformLayout()
at System.Windows.Forms.Control.ResumeLayout(Boolean performLayout)
at System.Windows.Forms.Layout.LayoutTransaction.Dispose()
at System.Windows.Forms.Control.SetVisibleCore(Boolean value)
at System.Windows.Forms.Control.set_Visible(Boolean value)
at ActiproSoftware.SyntaxEditor.EditorView.Dispose(Boolean disposing)
at ActiproSoftware.ComponentModel.DisposableObject.a(Boolean A_0)
at ActiproSoftware.ComponentModel.DisposableObject.Dispose()
at ActiproSoftware.SyntaxEditor.SyntaxEditor.c(a7 A_0)
at ActiproSoftware.SyntaxEditor.SyntaxEditor.Dispose(Boolean disposing)
at System.ComponentModel.Component.Dispose()
at ragScriptEditorShared.ScriptEditor.CloseFile(SyntaxEditor editor)
at ragScriptEditorShared.ScriptEditor.dockControl_Closing(Object sender, DockControlClosingEventArgs e)
at TD.SandDock.DockControl.OnClosing(DockControlClosingEventArgs e)
at TD.SandDock.DockControl.֩(Boolean ֪)
at TD.SandDock.DockControl.Close()
at ragScriptEditorShared.ScriptEditor.CloseFiles(List`1 files)
at ragScriptEditorShared.ScriptEditor.CloseAllFiles()
at ragScriptEditorShared.ScriptEditor.ExecuteAppAction(AppAction action)
at ragScriptEditor.Form1.closeAllTabsToolStripMenuItem_Click(Object sender, EventArgs e)
at System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e)
at System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e)
at System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e)
at System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e)
at System.Windows.Forms.ToolStripItem.FireEventInteractive(EventArgs e, ToolStripItemEventType met)
at System.Windows.Forms.ToolStripItem.FireEvent(EventArgs e, ToolStripItemEventType met)
at System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea)
at System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
at System.Windows.Forms.ToolStrip.WndProc(Message& m)
at System.Windows.Forms.ToolStripDropDown.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)