fix bug reported in #695: draw_region runs in a non-UI thread, so it …

…is possible for us to call it on a window that is being disposed of - also make a copy of the "self._backing" before dereferencing it

git-svn-id: https://xpra.org/svn/Xpra/trunk@7827 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/client/client_window_base.py

@@ -273,10 +273,17 @@ def refresh_all_windows(self):
 
     def draw_region(self, x, y, width, height, coding, img_data, rowstride, packet_sequence, options, callbacks):
         """ Note: this runs from the draw thread (not UI thread) """
-        assert self._backing, "window %s has no backing!" % self._id
+        if not self._backing:
+            log("draw_region: window %s has no backing, gone?", self._id)
+            from xpra.client.window_backing_base import fire_paint_callbacks
+            fire_paint_callbacks(callbacks, False)
+            return
         def after_draw_refresh(success):
             plog("after_draw_refresh(%s) %sx%s at %sx%s encoding=%s, options=%s", success, width, height, x, y, coding, options)
-            if success and self._backing and self._backing.draw_needs_refresh:
+            if not success:
+                return
+            backing = self._backing
+            if backing and backing.draw_needs_refresh:
                 self.queue_draw(x, y, width, height)
         callbacks.append(after_draw_refresh)
         self._backing.draw_region(x, y, width, height, coding, img_data, rowstride, options, callbacks)