Crash clicking combobox scrollbar quickly

[dacap]

On Aseprite v1.3-beta6 if we click the combobox scrollbar quickly we'll generate a crash because we access a deleted ui::Display* from a ui::Message:

 	Aseprite.exe!abort() Line 77	C++
 	[External Code]
>	Aseprite.exe!os::Window::pointToScreen(const gfx::PointT<int> & clientPosition) Line 34	C++
 	Aseprite.exe!ui::MouseMessage::screenPosition() Line 85	C++
 	Aseprite.exe!app::DynamicsPopup::onProcessMessage(ui::Message * msg) Line 480	C++
 	Aseprite.exe!ui::Widget::onProcessMessage(ui::Message * msg) Line 1539	C++
 	Aseprite.exe!ui::Widget::onProcessMessage(ui::Message * msg) Line 1539	C++
 	Aseprite.exe!ui::Widget::onProcessMessage(ui::Message * msg) Line 1539	C++
 	Aseprite.exe!ui::Widget::onProcessMessage(ui::Message * msg) Line 1539	C++
 	Aseprite.exe!ui::Manager::sendMessageToWidget(ui::Message * msg, ui::Widget * widget) Line 1916	C++
 	Aseprite.exe!ui::Manager::pumpQueue() Line 1778	C++
 	Aseprite.exe!ui::Manager::dispatchMessages() Line 834	C++
 	Aseprite.exe!ui::Manager::run() Line 291	C++
 	Aseprite.exe!app::App::run() Line 434	C++
 	Aseprite.exe!app_main(int argc, char * * argv) Line 105	C++
 	Aseprite.exe!wWinMain(HINSTANCE__ * hInstance, HINSTANCE__ * hPrevInstance, wchar_t * lpCmdLine, int nCmdShow) Line 41	C++

Reported:

• https://community.aseprite.org/t/aseprite-crashes-misbehaves-when-clicking-on-scroll-bar/10933
• https://igarastudio.zendesk.com/agent/tickets/2749