delphi access violation

Greetings

the following code gives a error:

"Project prKlanten.exe reaised exception class EAccessviolation with message 'acces violation at address 004d7767 in module 'prKlanten.exe'.
Read of address 0000005C."

procedure TfmOrder.FormCreate(Sender: TObject);
begin 
    dm.atDier开发者_运维技巧.Open;
    while not dm.atDier.eof do
    begin
       cbKeuze.Items.Add(dm.atDier['Diernaam']);
       dm.atDier.Next
    end;
      dm.atDier.Close;
    end;

Anyone know why? If you need more information feel free to ask.

[edit]

For more information see jasper's post

I bet that dm hasn't been created yet.

I bet that cbKeuse.Items hasn't been created yet.

I bet that dm hasn't been created yet.

Change this bit and run again:

[snip]
begin
    Assert(dm<>nil);
    Assert(dm.atDier<>nil); 
    dm.atDier.Open;
    while not dm.atDier.eof do
        [snip]

Do the assertions pass?

Access violations are caused when you access memory that is not yours such as:

• using a nil pointer
• buffer overrun
• accessing an uninitialized pointer/object
• accessing a freed pointer/object

Possible sources of access violations (or illegal/uninitialized pointers):

• dm is not set
• dm.atDier is not set
• cbKeuze is not set
• dm.atDier['Diernaam'] fails

In order to give more help, we need to known which line caused the access violation.

Addition Does the AV also happens if you move the code to a temporary OnClick of a temporary button? In other words, remove the code from the OnCreate as you can't be 100% sure the dm is fully operational. The OnActivate is another possibility, but you need to check for second activation.