Workaround to accessing the Revit API from other threads, see https://blog.dangl.me/archive/accessing-the-revit-api-in-callbacks-from-other-threads/

RevitApi_Correct.cs

var callbackStack = new Stack<Action>();

uiapp.Idling += (s, e) =>
{
    if (callbackStack.Any())
    {
        var action = callbackStack.Pop();
        action.Invoke();
    }
};

JavaScriptBridge
    .Instance
    .OnWebUIMessageReveived += (s, e) =>
    {
        callbackStack.Push(() =>
        {
            // You can safely access the Revit API here
        });
    };

RevitApi_Throws.cs

JavaScriptBridge
    .Instance
    .OnWebUIMessageReveived += (s, e) =>
    {
        // Accessing the Revit API here fails
    };