EnvDTE.DTE.Documents throwing everytime on first access, fine after

Question

I'm trying to store the opened documents from the Visual Studio environment. The code works for some solutions but I have 1 solution where it always throws an exception. I have tried to wait for some kind of initialization but nothing changes over time. As soon as I catch the exception, when I call the property again it's fine. What do I do wrong?

        internal static IList<Document> GetOpenedDocuments(DTE service)
        {
            ThreadHelper.ThrowIfNotOnUIThread();

            if (service == null)
            {
                throw new ArgumentNullException(nameof(service));
            }

            List<Document> openedDocuments = new List<Document>();

            foreach (Document document in service.Documents)
            {
                if (document != null)
                {
                    openedDocuments.Add(document);
                }
            }

            return openedDocuments;
        }

Accessing service.Documents always crashes the first time. Top of stack trace:

at EnvDTE.Documents.GetEnumerator() at XXX.FileHelper.GetOpenedDocuments(DTE service) in

Exception message:

The system cannot find the file specified. (Exception from HRESULT: 0x80070002)

Thanks for the help!