FIX: Closing an MDI Form Results in a General Protection Fault

MORE INFORMATION

Steps to Reproduce Problem

1. Start Visual Basic 4.0 or, if it is already running, click New Project on the File menu.
2. Set the MDIChild property of Form1 to True.
3. Add a ComboBox and two Command buttons to Form1. Set the ComboBox Style property to 1-Simple ComboBox.
4. Copy the following code to the Code window of Form1:

         Option Explicit
   
         Private Declare Function GetSystemMenu Lib "user32" _
                  (ByVal hWnd As Long, _
                  ByVal bRevert As Long) As Long
   
         Private Declare Function ModifyMenu Lib "user32" _
                  Alias "ModifyMenuA" _
                  (ByVal hMenu As Long, _
                  ByVal nPosition As Long, _
                  ByVal wFlags As Long, _
                  ByVal wIDNewItem As Long, _
                  ByVal lpString As Any) As Long
   
         Private Declare Function GetMenuItemID Lib "user32" _
               (ByVal hMenu As Long, _
               ByVal nPos As Long) As Long
   
         Const MF_BYPOSITION = &H400&
         Const MF_GRAYED = &H1&
         Const MF_BYCOMMAND = &H0&
   
         Sub Command1_click()
         ' Disables the Close button by calling a user-defined function.
            Dim hwndParent As Long
            hwndParent = Me.hWnd
            DisableItem hwndParent, "&Close", 6
         End Sub
   
         Sub DisableItem(hWnd As Long, _
                        sMenuCaption As String, _
                        iMenuPos As Integer)
         'User-defined function to disable the Close button on the
         'MDI Child Form toolbar.
            Dim hMenu As Long
            Dim hItem As Long
   
            hMenu = GetSystemMenu(hWnd, 0)
            hItem = GetMenuItemID(hMenu, iMenuPos)
            ModifyMenu hMenu, _
                        hItem, _
                        MF_BYCOMMAND Or MF_GRAYED, _
                        -9, _
                        sMenuCaption
         End Sub
   
         Private Sub Command2_Click()
             Unload Me
         End Sub
   
         Private Sub Form_Load()
             Me.Show
             Combo1.AddItem "first item"
             Command1.Caption = "Disable Close Button"
             Command2.Caption = "Unload MDI Child Form"
         End Sub
   
   						

5. From the Insert menu, click MDIForm to add an MDI form to the project. MDIForm1 is created by default.
6. On the Run menu, click start or press the F5 key to start the program. Select the first item in the ComboBox. Click the Close button in the Form1 form toolbar. An application error occurs with an error message.
7. Run the program again, click Disable Close Button. Check the Close menu item in the control menu to confirm that it is grayed out. Select the first item in the ComboBox, and then close the form using the Unload MDI Child Form button, the MDI Child form closes normally.