Closing a Smart Line results in a closed area "Shape" if "Join Elements" is selected. When you to touch the start point after placing some vertexes - the fill options underneath the Tool Settings Dialog pop up. As they appear you can not use the mouse to change the fill settings - they close immediately when the cursor is drawn away from the start point.
Using the "Arrows", or "Tab" on your Keyboard, scroll through the options in Tool Settings.
Use "Enter" to select and the arrows to browse.
You have to try around a bit for other options as they may require different handling.