C# Devexpress - Checked ComboBox strange behaviour

I have a checked ComboBox in my form:

What I want to do with the values of the checked values is to add them to the list of my temp object (that accepts a list of integers).

My problem is the following.

When I use this code (for testing purposes)

  foreach (object i in checkedComboBoxEdit1.Properties.GetItems().GetCheckedValues())
        {
           MessageBox.Show("Value:" + i.ToString() );
           //temp.intList.Add((int)i);
        }

I get what I want (a pop up for every checked item with the value of it)

However, when I pull my wanted method out of comment ( temp.intList.Add((int)i);), it throws an exception:

My other properties of my temp object are just fine. What is wrong with my code?

My object has a default constructor and the following property: List<Int> intList = new List<Int>()

What is wrong with my code?