what is object reference not set to an instance of an object

Question

In Windows Form , I Created ComboBox, when i try to Create a new name, it doesn't show any output. Instead of that it is showing " System.NullReferenceException : 'Object reference is not set to an instance of an object.

private void btnCountry_Click(object sender, EventArgs e)

{

MessageBox.Show(comboBox1.Text);

MessageBox.Show(comboBox1.SelectedItem.ToString());MessageBox.Show(comboBox1.SelectedIndex.ToString());

this.Close();

}

private void btnStates_Click(object sender, EventArgs e)

{

foreach(object obj in listBox1.SelectedItems)

{

MessageBox.Show(obj.ToString());

}

}