C# unable to clear combobox's selected index

See more:

C#

i got a combobox store list of item ( not using data bound ), a button used to clear all of the text of combobox and textbox, with a few textbox

when i press on the clear button, it show me this error message :

An unhandled exception of type 'System.NullReferenceException' occurred in bookbar user interface.exe

Additional information: Object reference not set to an instance of an object.

What I have tried:

this is my code

code for my button :

private void button1_Click(object sender, EventArgs e)
        {
            textBox1.Clear();
            textBox2.Clear();
            comboBox1.SelectedIndex = -1;
        }

i also set some code in the event of combobox, SelectedIndexChanged, this is where the error message show

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e) {
            if (comboBox1.SelectedItem.ToString() != "Lain-lain") { // < error in this line
                textBox1.Clear();
            }
            else  {
                textBox1.Enabled = true;
            }
            updatebooknameList();
        }