Word-Add in CreateDocoment gives AccessDenied

I've created a simple word plugin and want to create a document based off another document.

I've stripped down the code

function WordLoadDocument(msg) {
        Word.run(context => {
            var myNewDoc = context.application.createDocument();
            myNewDoc.properties.load();
            /*
            myNewDoc.properties.customProperties.add("fileId", file.id);
            myNewDoc.properties.customProperties.add("fileName", file.name);
            myNewDoc.properties.customProperties.add("attachId", attach.id);
            myNewDoc.properties.customProperties.add("attachName", attach.name);*/
            myNewDoc.open();
            return context.sync()
        }).catch(error => {
            if (error instanceof OfficeExtension.Error) {
                console.log(
                    "Error code and message: " + JSON.stringify(error.debugInfo)
                );

            console.log(error)

        }
    });

But I just get Error code and message: {"code":"AccessDenied","message":"AccessDenied","errorLocation":"Application.createDocument","statement":"var createDocument = v.createDocument();"...

It doesn't seem to matter what I do I just always get the error - even before I add in the data..

The manifest has ReadWriteDocument which seems to be the 'max' https://learn.microsoft.com/en-us/javascript/api/manifest/permissions?view=word-js-1.4

Is there something I'm missing. I'm on Word desktop and sideloading the app via network - could this be a permissions on the machine / type issue?

So I've worked this one out - basically the issue is that I had the add-in in a dialog box (displayDialogAsync). Word doesn't like it and hence the error.