Nil Object Exception on Window close

General
1

I am getting a Nil Object Exception on Window close which is very difficult to track down. When I step through the debugger, it just stops - and then when I hit Resume, it crashes.

If I place self.hide in the CancelClose event and then Return True, it doesn’t crash.

I am suspicious the problem is in the LostFocus event of an object but I can’t seem to trap the error.

I created a boolean property called WindowClosing and then set that to True in the CancelClose event, and then in objects with a LostFocus event I entered:

If WindowClosing then Return

at the beginning to hopefully stop this Nil Object Exception but I am still getting this error.

These are tough errors to track down. Are there any hints as to how to catch them?

Thanks.

2

What is the code in the Close event ?

1 Like
3

If there are any timers running on the window try explicitly turning them off in the close event.

1 Like
4

Are you using DataView ? Last version can potentially cause NilObject on windows close that Jim is working on…

5

I am using DataView however it is not being used on the window that is crashing. Does that matter?

6

I did try turning off all timers but it had no effect.

7 
If InvoiceModified then
  //s = the text you want displayed
  //sOK - what the OK button says
  //sCancel - what the Alternative button says
  //sExplanation = Additional text or information
  Dim i as Integer
  i = App.CONFIRM("Save Changes?","Save","Don't Save","","")
  If i = 1 then  //Save the Invoice
    If Check_for_Errors = False then
      sInvNumber = SaveInvoice
    Else
      Return True
    End If
  Elseif i = 2 then //Discard changes
    
  Else
    Return True
  End if
Else
  WindowClosing = True
  Timer_CheckGlobal.Mode = timer.ModeOff
  Timer_OptionKey.Mode = timer.ModeOff
End If
8

Should not…but I would suggest try with an older version, 1.15.5.2 is fine.

9

It is definitely related to having focus in a cell in a listbox. If I set focus to another control, the crash does not occur.

10

The ListBox is a Xojo ListBox or a DataView based one ?

11

It is a Xojo listbox.

12

It seems like the window close event is firing before the Listbox cell lostfocus event. Then when the lostfocus event fires, it no longer exists.

13

It looks like I solved the problem by changing focus to a different control in the window close event.

This code works:

If InvoiceModified then
  //s = the text you want displayed
  //sOK - what the OK button says
  //sCancel - what the Alternative button says
  //sExplanation = Additional text or information
  Dim i as Integer
  i = App.CONFIRM("Save Changes?","Save","Don't Save","","")
  If i = 1 then  //Save the Invoice
    If Check_for_Errors = False then
      sInvNumber = SaveInvoice
    Else
      Return True
    End If
  Elseif i = 2 then //Discard changes
    
  Else
    Return True
  End if
Else
  WindowClosing = True
  fldDate.SetFocus
End If
1 Like
14

I’ve got this EXACT SAME problem but not sure how to implement the “fix” noted here. I have a modal window with a few listboxes (actually DataView) and a few buttons inside the main window. When I edit a cell in the listbox, the program crashes (“An exception of NilObjectException was not handled. The application must shut down.”) when I try to close the modal window.

Any suggestions?

Windows 10
Xojo 2016 r1.1

15

While I have never encountered this, my interpretation of the replies by @David_Schlam is that you just need to shift the focus to outside the listbox / dataview prior to the close event. So assuming you have another control that can accept focus (e.g. an enabled text field), try adding a CancelClose event to the window and in it place a TextField1.SetFocus, where TextField1 is the name of some control that can accept the focus on each OS.

16

Thanks for taking the time to respond, Douglas. Unfortunately, I interpreted the “fix” the same way as you and had already tried putting a textfield on the modal window to allow TextField1.SetFocus in the CancelClose … No Love!!!