Access violation while changing the theme

[tim-lebedkov]

I updated Virtual TreeView to 7.6.5a and started getting this exception while changing the theme from default Windows to "Light". The program is compiled in XE5.

[0000000000E05382] VirtualTrees.StyleHooks.TVclStyleScrollBarsHook.Destroy (Line 520, "VirtualTrees.StyleHooks.pas")
[000000000040E4C8] System.TObject.Free
[0000000000FFA876] Vcl.Styles.TStyleEngine.DoRemoveControl
[0000000000FFADBF] Vcl.Styles.TStyleEngine.Notification
[00000000007E6F7F] Vcl.Themes.TStyleManager.Notification
[00000000007B0D79] Vcl.Controls.TWinControl.WMDestroy
[000000000040EBD1] System.TObject.Dispatch
[00000000007A6566] Vcl.Controls.TControl.WndProc
[00000000007ADC33] Vcl.Controls.TWinControl.WndProc
[0000000000E706AF] VirtualTrees.TBaseVirtualTree.WndProc (Line 25893, "VirtualTrees.pas")
[00000000007ACE6C] Vcl.Controls.TWinControl.MainWndProc
[0000000000552A36] System.Classes.StdWndProc
[00007FF8BD45E858] Unknown function at CallWindowProcW
[00007FF8BD45E3DC] Unknown function at DispatchMessageW
[00007FF8BD470C93] Unknown function at SendMessageTimeoutW
[00007FF8BEDF0E64] KiUserCallbackDispatcher
[00007FF8BC8C2384] NtUserDestroyWindow
[00000000007AC580] Vcl.Controls.TWinControl.DestroyWindowHandle
[00000000007AC514] Vcl.Controls.TWinControl.DestroyWnd
[00000000007AC8F2] Vcl.Controls.TWinControl.DestroyHandle
[00000000007AC8AA] Vcl.Controls.TWinControl.DestroyHandle
[0000000000757D30] Vcl.Forms.TCustomForm.DestroyHandle
[00000000007AC978] Vcl.Controls.TWinControl.CMRecreateWnd
[000000000040EBD1] System.TObject.Dispatch
[00000000007A6566] Vcl.Controls.TControl.WndProc
[00000000007ADC33] Vcl.Controls.TWinControl.WndProc
[0000000000754960] Vcl.Forms.TCustomForm.WndProc
[0000000000D6F568] JvDockControlForm.TJvDockBaseControl.WindowProc (Line 2484, "JvDockControlForm.pas")
[0000000000D722C7] JvDockControlForm.TJvDockClient.WindowProc (Line 3584, "JvDockControlForm.pas")
[00000000007A6042] Vcl.Controls.TControl.Perform
[0000000000754850] Vcl.Forms.TCustomForm.WndProc
[0000000000D6F568] JvDockControlForm.TJvDockBaseControl.WindowProc (Line 2484, "JvDockControlForm.pas")
[0000000000D722C7] JvDockControlForm.TJvDockClient.WindowProc (Line 3584, "JvDockControlForm.pas")
[00000000007ACE6C] Vcl.Controls.TWinControl.MainWndProc
[0000000000552A36] System.Classes.StdWndProc
[00007FF8BD45E858] Unknown function at CallWindowProcW
[00007FF8BD45DE1B] Unknown function at SendMessageW
[00007FF8BD45D68A] SendMessageW
[00000000007E7521] Vcl.Themes.TStyleManager.SetStyle
[00000000007E7A2A] Vcl.Themes.TStyleManager.TrySetStyle

[tim-lebedkov]

In the following method FVertScrollWnd is nil, but Self is not nil:

destructor TVclStyleScrollBarsHook.Destroy;
begin
  FVertScrollWnd.StyleHook := nil;
  FreeAndNil(FVertScrollWnd);
  FHorzScrollWnd.StyleHook := nil;
  FreeAndNil(FHorzScrollWnd);
  inherited;
end;

[joachimmarder]

Well, I no more have a Delphi XE5 at hand, but I remember that at least in XE2 - XE5 the VCL style code was not the best. Besides updating to a newer Delphi version, you can try if simply adding assign checks solves the problem:

destructor TVclStyleScrollBarsHook.Destroy;
begin
  if Assigned(FVertScrollWnd) then
  begin
    FVertScrollWnd.StyleHook := nil;
    FreeAndNil(FVertScrollWnd);
  end;
  if Assigned(FHorzScrollWnd) then
  begin
    FHorzScrollWnd.StyleHook := nil;
    FreeAndNil(FHorzScrollWnd);
  end;
  inherited;
end;

If so, I will add this change.

[tim-lebedkov]

The change helps. Thank you.