Can Form.Show() make the form itself become null after the call?

Question

This is really really the most annoying thing in my programming life:

1. I have a form which is kept alive along with my application. I have no code calling to some method which will dispose it, I don't want it to be disposed by any reason, and I'm sure if it happens, the reason would not be in my code. This form is a custom form which has a method called Next(), this Next() simply displays the next item info on the form.

2. The worst thing occurred when I pressed on a button which caused the call to Next() and there was an exception saying "Object reference not set to an instance of an object". What? And here is the code, I saved a reference of my form into a Form variable called "currentShownForm" (there is only 1 of my forms are shown at a time, all of these should live along with my application):

   private void ShowForm(CustomRibbonForm form)
   {               
       if (!form.Visible)
       {
           if (currentShownForm != null) currentShownForm.Hide();
           form.Show();                
           currentShownForm = form;                
       }
   }
   

and the Next() is called like this if I press the button Next:

currentShownForm.Next();

The enigma is in the form.Show(); With the method ShowForm() above, do you think the currentShowForm can be null??? What? Any case? The only moment it is null is before the first form is shown. After the first is shown, it will be the first form, any other next form is shown will be referred by it. So how it becomes null in some case??? The code in ShowForm() method is the only one can change the reference of currentShownForm in my project.

That really baffled me a lot, I couldn't believe in what I saw. Oh my God, I didn't have any clue on this, any idea, but I had to try the most ridiculous thing like this:

private void ShowForm(CustomRibbonForm form){
        if (!form.Visible)
        {
            if (currentShownForm != null) currentShownForm.Hide();
            currentShownForm = form; 
            form.Show();                                               
        }
}

Wow, before trying this, I didn't thing it could make any thing different, but it did work. The currentShownForm was not null when I pressed the Next button. What is the magic of the swap between the two lines of code in ShowForm??? I think of only 1 reason for that, the form.Show() somehow disposed the form itself? But why was it still shown?

The noticeable thing is my form is not .NET form (just inherits from .NET form), it is a custom form, and again it's from DotNetBar lib, wow, I have an idea about writing a book of the most annoying things you should know when working with DotNetBar (At least, I've had about 5 things to write, I bet there will be more if I still use it).

Please see the code above and give me some explanation on why it could happen that way? I can't explain it, in fact, I suppose that's a bug.

Your help would be highly appreciated!

Thank you!

UPDATE

Now, at least I know the original thing from which the problem occurs but still don't understand why:

In fact my form has some Custom controls, the involved control here is a CustomComboBox. This comboBox is a 2-in-1 control consisting of a normal ComboBox (again, a DotNetBar combobox namely ComboBoxEx) and a focusable Label (a custom UserControl). All the controls register a ParentChanged event to register some events for their parent form, like this:

protected override void OnParentChanged(EventArgs e){
  if(Parent is CustomRibbonForm){
      ((CustomRibbonForm)Parent).RefreshControls += (s,ev) => {
              Show();
              Hide();
      };
  }   
}

My form has a defined event called RefreshControls which will be raised when needed. It has a public method to raise that event called RefreshAllControls. And the call to that method is placed after ShowForm() in the Activated event handler for my main form (not the form I'm talking about) like this:

Activated += (s,e) => {
    ShowForm(myForm);
    myForm.RefreshAllControls();
};

And here is what next: When I comment out the Show(); and Hide(); in the OnParentChanged method, it works OK in both cases:

protected override void OnParentChanged(EventArgs e){
  if(Parent is CustomRibbonForm){
      ((CustomRibbonForm)Parent).RefreshControls += (s,ev) => {
              //Show();
              //Hide();
      };
  }   
}

But if not, only 1 case works (the currentShowForm is assigned before the call to form.Show(). It's closer to the cause to the problem but it's still not easy to understand.

Hope you have something to explain to me with this update! Thank you

Next UPDATE

Now I know the currentShownForm is null because the first assignment was in fact never done, the form is shown but the currentShownForm = form is not executed and because currentShownForm is initially null, it's still null after the call to ShowForm() method.

It's very close to the cause but it's still very strange:

private void ShowForm(CustomRibbonForm form){
        if (!form.Visible)
        {
            if (currentShownForm != null) currentShownForm.Hide();
            form.Show(); //<------ I marked a break point here.                                               
            currentShownForm = form; //<----- I also marked a break point here.
        }
}

After marked two break points following each other in the same method, it seems to be sure that all the break points will be stepped through, but only the first is stepped through. After that, the main UI is shown and the second break point is bypassed???????? What?????????????? There are many of events occurred after the call to form.Show(), most of them are events in my Custom controls (relating to Focus, LostFocus and Paint events). This is the first time I've seen such a strange code stepping. Now this is very close to the cause, hope you have something to say. Thank you