You haven't constructed the controls yet!
Either call InitializeComponent inside all your constructors, or reference the default constructor that does have it first by using construction chaining:
public partial class New_Invoice : DevExpress.XtraEditors.XtraForm
string getOper = "A";
public New_Invoice(string oper, int invoiceno) : this()
textEdit5.Text = invoiceno.ToString();
textEdit5.Visible = false;
getOper = oper;