NullReferenceException: On SelectionChangeCommitted and SelectedIndexChanged

NullReferenceException: On SelectionChangeCommitted and SelectedIndexChanged:
While working with ComboBox in .Net windows form, I got an error

System.NullReferenceException:

Object reference not set to an instance of an object. at SP_Script_Generator.Form1.procedureType_SelectedIndexChanged(Object sender, EventArgs e)