Function: Unable to add top level control to resolution in control

Scenario Description : When you are working on a project, you need to operate on two interfaces at the same time, so you use the TabControl control when you add form forms to TabPage

Error: Unable to add top level control to resolution in control

Workaround :

                        Frm_checkerr = new Nwchk2err.frmcheckerror ();   
                        Frm_checkerr.xmlinfile = Xmlinparam;
                        Frm_checkerr.onshowendreslut + = Frm_check_onshowendreslut;
                        FRM_CHECKERR.TRIGJTBC + = FRM_CHECKERR_TRIGJTBC;
                        TABORDER1.CONTROLS.ADD (frm_checkerr);
                        Frm_checkerr.toplevel = false;               Set to non-top-level control
                        frm_checkerr.mdiparent = this;               Adding the basic error
                        TABORDER1.CONTROLS.ADD (frm_checkerr) in parallel;        Add Interface
                        //splitcontainer1.panel2.controls.add (tabOrder1) in TabPage;
                        Frm_checkerr.show ();

Note : You must initialize before use, the newly created Form object can set toplevel already used form (show or open) cannot use toplevel, what is the top-level control to be studied