Stokes demo broken in adaptive case

[jwallwork23]

Running python3 opt_hessian.py stokes --debug on branch https://github.com/mesh-adaptation/opt_adapt/tree/24_pyroteus-migrate gives the following error:

Using method gradient_descent
  1:  J = 4.6078e+01,  ||dJdm|| = 1.1935e+02,  step length = 1.0000e-03,  #elements =   746,  time = 0.67s
Traceback (most recent call last):
  File "/home/joe/software/Firedrake/firedrake-nov24/src/opt_adapt/demos/opt_hessian.py", line 99, in <module>
    m_opt = minimise(
  File "/home/joe/software/Firedrake/firedrake-nov24/src/opt_adapt/opt_adapt/opt.py", line 555, in minimise
    for o in step(it, forward_run, mesh, params, *args):
  File "/home/joe/software/Firedrake/firedrake-nov24/src/opt_adapt/opt_adapt/opt.py", line 222, in _gradient_descent
    u += lr * P
  File "/home/joe/software/Firedrake/firedrake-nov24/src/firedrake/firedrake/adjoint_utils/function.py", line 146, in wrapper
    func = __iadd__(self, other, **kwargs)
  File "/home/joe/software/Firedrake/firedrake-nov24/src/firedrake/firedrake/function.py", line 509, in __iadd__
    IAddAssigner(self, expr).assign()
  File "/home/joe/software/Firedrake/firedrake-nov24/src/firedrake/firedrake/assign.py", line 147, in __init__
    expression = as_ufl(expression)
  File "/home/joe/software/Firedrake/firedrake-nov24/src/ufl/ufl/constantvalue.py", line 517, in as_ufl
    raise ValueError(
ValueError: Invalid type conversion: [Product(FloatValue(7.35516390623806e-05), Indexed(Coefficient(WithGeometry(FunctionSpace(<firedrake.mesh.MeshTopology object at 0x7715b1bb04f0>, VectorElement(FiniteElement('Lagrange', triangle, 2), dim=2), name=None), Mesh(VectorElement(FiniteElement('Lagrange', triangle, 1), dim=2), 117)), 323), MultiIndex((FixedIndex(0),))))
 Product(FloatValue(7.35516390623806e-05), Indexed(Coefficient(WithGeometry(FunctionSpace(<firedrake.mesh.MeshTopology object at 0x7715b1bb04f0>, VectorElement(FiniteElement('Lagrange', triangle, 2), dim=2), name=None), Mesh(VectorElement(FiniteElement('Lagrange', triangle, 1), dim=2), 117)), 323), MultiIndex((FixedIndex(1),))))] can not be converted to any UFL type.