private void MetroWindow_ContentRendered(object sender, EventArgs e)
        {
            PageAdditional p = (PageAdditional)pageWithButton.Content;

            if (p == null)
            {
                return;
            }
            p.OnMessageShow += P_OnMessageShow;
        }
        // it isn't working
        private async void P_OnMessageShow(object sender, EventArgs e)
        {
            var result = await this.ShowInputAsync("Test", "Test message isn't showing");

            this.ShowModalMessageExternal("Test", "Message from page was showed");

            PageAdditional p = (PageAdditional)pageWithButton.Content;

            if (p == null)
            {
                return;
            }
            p.MessageReceived(this, e);
        }