How do I show a child form from an MDI form

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C#

WinForms

MDI

Hi,
I have a main form in Visual Studio C# which has the IsMdiContainer set to True.
I tried to open a child form using

frm_edit_s = new frm_edit_sensor();
frm_edit_s.FLG_parent_form = this;
frm_edit_s.MdiParent = this;
frm_edit_s.FormClosed += new FormClosedEventHandler(frm_video_closed);
frm_edit_s.BringToFront();
frm_edit_s.Visible = true;
frm_edit_s.Show();

When the code is executed, I can step through the code but the child form is not displayed. It seems to be there as the FormClosed event fires when the program is stopped.
If I remove the

frm_edit_s.MdiParent = this;

line, the form is displayed
On searching , it seems to be related to MenuStrip which is used in the parent form though I cannot find a solution. Any advice appreciated
Regards

What I have tried:

I have tried launching the child from from a button rather than from the menustrip
as well as making sure the form is visible and at the front