Created attachment 80743 [details]
GDB backtrace

Build the custompanel example from the SDK.
Run the example.
Attach the debugger.

Set two break points:
a) on the UI element's disposing()
sd::colortoolpanel::PanelUIElement::disposing()

b) on its tool panel disposing()
sd::colortoolpanel::SingleColorPanel::disposing()

Activate the custompanel on the sidebar, switch to the gallery or the navigator panel.

sd::colortoolpanel::SingleColorPanel::disposing() is called before sd::colortoolpanel::PanelUIElement::disposing()

sfx2::sidebar::Panel::Dispose() is disposing the UI element's real interface, then the UI element itself:


    if (mxElement.is())
    {
        Reference<lang::XComponent> xComponent (mxElement->getRealInterface(), UNO_QUERY);
        if (xComponent.is())
            xComponent->dispose();
    }

    {    
        Reference<lang::XComponent> xComponent (mxElement, UNO_QUERY);
        mxElement = NULL;
        if (xComponent.is())
            xComponent->dispose();
    }

The real interface of the UI element is the UI element's responsibility (this means, when the UI element is disposed, it will free its resources, among other things, disposing its real interface - whatever that could be).
In the custompanel example, this is handled nicely by the C++ component implementation helper class. In other cases, it may end up in a crash, as with the Watching Window extension from http://extensions.openoffice.org/en/project/watchingwindow