Object reference not set to an instance of an object during xml serialization and problem with selection of combobox item upon loading - wpf

Question

question 1. I have this issue of "Object reference not set to an instance of an object" when my Majorlabel is empty and this occurs after i try to do a save button click on xml serialization. How can i fix this?
private void SaveButton_Click(object sender, RoutedEventArgs e)
{
string savepath;
SaveFileDialog DialogSave = new SaveFileDialog();
// Default file extension
DialogSave.DefaultExt = "txt";
// Available file extensions
DialogSave.Filter = "XML file (*.xml)|*.xml|All files (*.*)|*.*";
// Adds a extension if the user does not
DialogSave.AddExtension = true;
// Restores the selected directory, next time
DialogSave.RestoreDirectory = true;
// Dialog title
DialogSave.Title = "Where do you want to save the file?";
// Startup directory
DialogSave.InitialDirectory = #"C:/";
DialogSave.ShowDialog();
savepath = DialogSave.FileName;
DialogSave.Dispose();
DialogSave = null;
FormSaving abc = new FormSaving();
if (!string.IsNullOrEmpty(MajorversionresultLabel.Content.ToString()))
{
abc.Majorversion = MajorversionresultLabel.Content.ToString();
}
abc.Startzbuildfrom = StartzbuildcomboBox.SelectedItem.ToString();
using (Stream savestream = new FileStream(savepath, FileMode.Create))
{
XmlSerializer serializer = new XmlSerializer(typeof(FormSaving));
serializer.Serialize(savestream, abc);
}
}
As recommended,
here is the line of error:
if (!string.IsNullOrEmpty(MajorversionresultLabel.Content.ToString()))
{
abc.Majorversion = MajorversionresultLabel.Content.ToString();
}
Question 2. I used this line to save my combo box selection:
abc.Startzbuildfrom = StartzbuildcomboBox.SelectedItem.ToString();
and in my load i have this line:
StartzbuildcomboBox.SelectedItem = abc.Startzbuildfrom
why wont it select the combobox selection previously?

Answer 1

As a first note, I'd recommend only putting one question into a single query here. Makes it easier.
For your second question, my guess is that you're running into a reference variable problem. I think that calling the ToString() method on the SelectedItem actually creates an entirely new string variable. Then, when you try to set the selected item later, it can't find the new string as a possible item to select because, even though the two strings have the same value, they are different objects. I would maybe recommend that you either:
1) Set the selected item by searching through your combo box contents to find a string whose value matches the one you've saved
or
2) Save the actual reference by saying abc.Startzbuildfrom = StartzbuildcomboBox.SelectedItem. Then set the selected item from that reference.

Answer 2

I suspect that MajorversionresultLabel is null, or MajorversionresultLabel.Content is null. Thus your statement
if (!string.IsNullOrEmpty(MajorversionresultLabel.Content.ToString()))
will throw a NullReferenceException. Try this instead:
if (MajorversionresultLabel != null && MajorversionresultLabel.Content != null && MajorversionLabel.Content.ToString() != string.Empty)
I bet your NullReferenceException will go away.