Empty form raises on ufl_cell()

[Jan Blechta]

Following preprocessed form is empty and attempt to get the cell raises uniformative

IndexError: tuple index out of range

Is it what we want? Should it return original cell or None or raise an informative message?

from ufl import *
from ufl.algorithms import compute_form_data

Q1 = VectorElement("CG", triangle, 1)
V1 = FiniteElement("N1div", triangle, 1)
vec = VectorConstant(triangle)
q1 = Coefficient(Q1)
v1 = Coefficient(V1)
F = derivative((q1-vec)**2*dx, v1)

fd = compute_form_data(F)
fd.preprocessed_form.ufl_cell()