Answer:
In Chapter 7.2.1 of the SHAPE‑THIN manual, you can find the formulas for shear areas and Bredt's torsional moment of inertia. There, the element thickness t* is in the denominator, so division by zero would be a problem. For "free" null elements, you should define an equivalent thickness; for example, the thickness of one of the adjacent elements.
Select the null element and activate the "Effective Shear Thickness" option in the "Edit Element" dialog box. The t* text box becomes accessible. Then the calculation is possible.
