First, I hope this is not another typo, but you have:
if ((isClosing == false)&&(comboBox1.SelectedValue=null))
Without the double equal sign, comboBox1.SelectedValue will be set to null.
That would result in your call
String cb1Value = comboBox1.SelectedValue.ToString();
raising an object reference not set to an instance of an object error.
As far as comboBox2 not being reset, I'm not sure. Setting the SelectedIndex to -1 should do that. Though, you really don't need it in a try/catch block.
Setting SelectedIndex = -1 will never throw an error unless comboBox2 has been set to null at some point.