Предотвратить повторное создание формы

NotWorkButton corOutForm = new NotWorkButton();
corOutForm.MdiParent = this;
corOutForm.WindowState = FormWindowState.Maximized;
corOutForm.Show();

internal static bool isCreated = false;

public Form(){
   if(Form.isCreated == true){
      return; //кажется так, либо по-другому выходить из метода
   }
   InitializeComponents();
}

private Form_Closed(object sender, ...){
   Form.isCreated = false;
}

if (_corOutForm == null){
   _corOutForm = new NotWorkButton();
}
...

createNewFormButtonName.Enabled = false;

createNewFormButtonName.Enabled = true;

...
SubForm _subForm;
static bool is_subForm = false; 
 
public static bool Is_subForm
{
    set
    {
        is_subForm = value;   
    }
}
 
...
private void SubMenuItem_subForm_Click(object sender, EventArgs e)
{
    if (!is_subForm)
    {
        _subForm = new SubForm();
        _subForm.MdiParent = this;
        _subForm.WindowState = FormWindowState.Maximized;
        _subForm.Show();
        is_subForm = true;
    }
    else
    {
        _subForm.WindowState = FormWindowState.Maximized;    
    }
}

private void NotWorkButton_FormClosing(object sender, FormClosingEventArgs e)
{
    MainForm.Is_subForm = false;
}