As we talked in your support ticket, the problem is fixed in the latest version. Catching this concrete exception will lead to ProofingErrosDecorationUILayer not working correctly. Once a new character is added to the document or any other operation that triggers the update of the editor layout is executed the ProofingErrosDecorationUILayer will have the correct context and should work as expected.
Another possible approach is the ProofingErrosDecorationUILayer to be inherited and in the UpdateUIViewPortOverride method the delayed execution to be removed. If you choose this path I recommend you to read this article about Customize Presentation through UI Layers
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