public void ContinueToLexiconView()
        {
            if (LexiconViewModel == null)
            {
                LexiconViewModel = new LexiconViewModel();
            }

            RunOnUIThread(() => _navigator.NavigateTo <LexiconViewModel>());
        }
        public void ContinueToRootView()
        {
            if (RootViewModel == null)
            {
                RootViewModel = new RootViewModel();
            }

            if (LexiconViewModel == null)
            {
                LexiconViewModel = new LexiconViewModel();
            }

            if (NewWordViewModel == null)
            {
                NewWordViewModel = new NewWordViewModel();
            }

            if (LearningViewModel == null)
            {
                LearningViewModel = new LearningViewModel();
            }
            //LearningViewModel.LoadData ();

            if (RandomTestViewModel == null)
            {
                RandomTestViewModel = new RandomTestViewModel();
            }

            if (IntroductionViewModel == null)
            {
                IntroductionViewModel = new IntroductionViewModel();
            }

            RunOnUIThread(() => {
                _navigator.NavigateTo <RootViewModel> ();
            });
        }