An exception of type 'System.Reflection.TargetInvocationException' occurred in System.Windows.ni.dll windows phone 8

Question

I am building a note taking app. on the note editing page, when users create new note and types in text, and then press the phone's back button, the code is suppose to create a new note and i try to replace carriage returns with new line. when i try this in the app i get this exception.An unhandled exception of type 'System.Reflection.TargetInvocationException' occurred in System.Windows.ni.dll and a new file tab opens saying System.Windows.pdb not loaded This is the code where it breaks

protected async override void OnNavigatedFrom(NavigationEventArgs e)
    {
        base.OnNavigatedFrom(e);

        // we are suppose to save the note, when user navigates away from the page
        ourNote.Note = editorTextBox.Text.Replace("\r", "\n");
        await App.ViewModel.UpdateNotes();
    }

does anyone know what i'm getting wrong? this is the NoteModel class, the ourNote variable is of this type.

public class NoteModel : INotifyPropertyChanged
{

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(String propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (null != handler)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    private string _id;

    public string ID 
    {
        get 
        { 
            return _id;
        }
        set 
        {
            // check to see if the value isn't the same with _id if not raise a propertychanged event via the method
            if (value != _id)
            {
                _id = value;
                NotifyPropertyChanged("ID");
            }
        } 
    }

    private string _modDate;

    public string ModifiedDate
    {
        get
        {
            return _modDate;
        }
        set
        {
            // check to see if the value isn't the same with _id if not raise a propertychanged event via the method
            if (value != _modDate)
            {
                _modDate = value;
                NotifyPropertyChanged("ModifiedDate");
            }
        }
    }

    private string _note;

    public string Note
    {
        get
        {
            return _note;
        }
        set
        {
            // check to see if the value isn't the same with _id if not raise a propertychanged event via the method
            if (value != _note)
            {
                _note = value;

                string[] lines = _note.Split('\n'); // we change this from '\r' to '\n' because of winRT serializaiton and deserialization ends up with line feeds only, removes carriage returns
                if (lines.Length > 0)
                {
                    FirstLine = lines[0]; // we removed .Replace("\n", ""); since no more carriage returns
                }
                else
                {
                    // what if there is no first line
                    FirstLine = "(empty)";
                }
                NotifyPropertyChanged("Note");
            }
        }
    }

    private string _firstLine;

    public string FirstLine
    {
        get
        {
            return _firstLine;
        }
        set
        {
            // check to see if the value isn't the same with _id if not raise a propertychanged event via the method
            if (value != _firstLine)
            {
                _firstLine = value;
                NotifyPropertyChanged("FirstLine");
            }
        }
    }
}

Answer 1

I think the exception occurred because some elements of your notes list page (the page when you back) aren't loaded.

Does App.ViewModel.UpdateNotes() update a element of your notes list page? If yes, you should only update data, and then update notes list page element when you OnNavigateTo the page.