scene2d example assert when releasing mouse outside of window.

Error is : ASSERT: "pickTriangle->entity()" in file items\scene2d.cpp, line 484

So it seems that when releasing the mouse outside of the window, pickTriangle->entity() is null.

line in source

Step to reproduce:

1. Start scene2d example
2. Middle click on scene2d to make it stop
3. Drag slider with mouse out of the window
4. Release outside of the window

I put a gif as attachement.