It looks more like a scoping problem by looking at line '_Main = new frmMDI();'
follow these steps:
- create a class named 'ReferenceTable'
- create a static variable named _Main in ReferenceTable
- set ReferenceTable._Main = new frmMain(); // in Program.cs
- set childform.Parent = ReferenceTable._Main //in all your child form code before calling Show() or showDialog() methods