What's this error?

[alchimiste]

Hi, everybody...

It's very hot summer in Korea

I've received the error report from a user.

Exception: Tried to get a Renderer2d for a rendering surface that no longer exists. Did you try to draw or query a Renderer2d for a View, Drawable or Window that has been destroyed?
#0 Renderer2dProxy.renderer2d-target (getter) (0x05a38e50-72)
#1 Renderer2dProxyControl.renderer (getter) (0x05a51377-21)
#2 PartialUpdateWindow.on-repaint-window-event (0x05a5131f-69)
#3 PartialUpdateWindow.internal-handle-event (0x05a4da46-102)
#4 Window.handle-event (0x05a4d994-84)
#5 WindowEvent.fire-self (0x05a512bb-23)
#6 RepaintWindowEvent.fire (0x05a51206-84)
#7 Window.update-window (0x05a510f9-113)
#8 Window.update-windows (0x05a5100f-311)
#9 Win32EventCheckerHandler.last (0x05a50ebc-26)
#10 EventChecker.filter-or-wait-or-next-or-last (0x04e43424-950)
#11 EventQueue.dispatch-events-aux (0x04e4bb38-208)
#12 EventQueue.dispatch-events (0x04e4b9b5-157)
#13 dispatch-events (0x04e4b8db-59)
#14 run-applet (0x05f3bbc6-174)
#15 ProcessRunnable.run-bound-proc (0x04e3ea13-1235)
#16 ProcessRunnable.run (0x04e3e53a-16)
#17 curl-thread-really-really-call-run-in-runnable (0x04e3e510-24)
#18 curl-thread-really-call-run-in-runnable (0x04e39977-239)

I made the application that uses Renderer2d.

The error does not happen in all users' computers.

What's the cause of it and What do I do for it?