MDI Application - Cannot open child form

I made 2 form. MainForm and ChildForm(I set become available forms)

design >> https://i.sstatic.net/R4XWf.png

when I want to call my ChildForm use this code

  void __fastcall TMainForm::ChildForm1Click(TObject *Sender)
    {
      if(!ChildForm)
       { ChildForm =  new TChildForm(this); }
      else
       { ChildForm->WindowState=wsNormal; }
    }

and to close

void __fastcall TChildForm::FormClose(TObject *Sender,
  TCloseAction &Action)
 {
    Action=caFree;
 }

why when i open ChildForm then i close. and when to open again . ChildForm Can't open ??

Main Form

#include <vcl.h>
#pragma hdrstop

#include "Unit1.h"
#include "Unit2.h" //ChildForm
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TMainForm *MainForm;
//---------------------------------------------------------------------------
__fastcall TMainForm::TMainForm(TComponent* Owner)
    : TForm(Owner)
{
}
//---------------------------------------------------------------------------
void __fastcall TMainForm::ChildForm1Click(TObject *Sender)
{
 if(!ChildForm)
{ ChildForm =  new TChildForm(this); }
else
{ ChildForm->WindowState=wsNormal; }
}

Solution

As it has been discussed in the comments, and thanks to the help of @Remy Lebeau, in the TMainForm class, you need to set the ChildFrom pointer to NULL after having destroyed the form it points to. This may be accomplished in the destructor of the TChildForm class, by using the pointer of its owner that has been passed to it at construction time.

The issue is basically in that code snippet:

  if(!ChildForm)
   { ChildForm =  new TChildForm(this); }
  else
   { ChildForm->WindowState=wsNormal; }

The first time, everything works as planned. When ChildForm is destroyed however, the pointer isn't reset, and the next time ChildForm creation is requested, the first test fails, and of course the form isn't created.

So:

make the TChildForm class keep a copy of its owner's pointer (you'll probably need to change the constructor prototype to the following:
```
__fastcall TChildForm::TChildForm(TMainForm *owner)
```
in order for TChildForm to know the full type of its owner (the IDE should #include the TMainForm header for you, or ask for the permission to do it).
Create a method in TMainForm to handle TChildForm pointer reset, and give it the right access level for a TChildForm to invoke it.
add a call to that method (ie. owner->onChildFormTermination() or something like that) in ~TChildForm to let its owner know.

Note that my VCL-fu is a bit rusty (I haven't coded with C++Builder for 10+ years), there might be a more idiomatic way of writing this.