There's no such thing as a "closing tool". SolveSpace considers any contour with exactly one cycle a "closed contour". This is also the only kind of contour that is usable with 3d operations like extrusion, which is why a message is shown when the condition isn't satisfied.

See an attachement. It has 2 closed contours, but the program say they aren't.

If extrusion needs some conditions, namely an application of extrusion tool should fail or warn. No need to indicate any errors if exctrusion tool was not called. Also no need to fail area calculation tool.