NullReferenceException in VisualStudio2005SyntaxEditorRender

I haven't been to repro this bug that one of my users reported, but I think it's important to post it nonetheless.

We use something other than your UIStudio to handle our dockable windows. Our SyntaxEditor controls are placed on a TabbedDocument to look just like Visual Studio. When someone right-clicks a TabbedDocument's tab, a context menu appears wtih "New Tab", "Close Tab", "Close Other Tabs", and "Close All Tabs" options.

My user had 2 documents open and when he tried to close the one that was not in focus, this happened:

System.NullReferenceException: Object reference not set to an instance of an object.
at ActiproSoftware.SyntaxEditor.VisualStudio2005SyntaxEditorRenderer.MeasureLineNumberMarginWidth(Graphics g, SyntaxEditor syntaxEditor)
at ActiproSoftware.SyntaxEditor.EditorView.MeasureCore(Graphics g, Size availableSize)
at ActiproSoftware.WinUICore.UIElement.Measure(Graphics g, Size availableSize)
at ActiproSoftware.WinUICore.UIControl.MeasureCore(Graphics g, Size availableSize)
at ActiproSoftware.WinUICore.UIControl.Measure(Graphics g, Size availableSize)
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(a8 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.A(Boolean )
at TD.SandDock.DockControl.Close()
at ragScriptEditor.Form1.closeTabToolStripMenuItem_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)

Right-clicking the tab for the document that is not in focus, brings it into focus (and hides the other document). A quick succession of visibility changes could contribute to the problem, I imagine. It goes something like this:

1) With Doc A in focus, the user right-clicks Doc B
2) Doc A is hidden
3) Doc B is made visible
4) Doc B is closed
5) Doc A is made visible

I'm not sure where in those 5 steps the exception ocurred, probably step 3 or 5. If I can get specifics from my user, I will let you know.

EDIT: My user said the crash ocured somewhere between step 3 and 4, perhaps on 4. Doc B's tab had not yet been removed, at least visually.

Thanks!

[Modified at 10/26/2007 07:41 PM]