Hello!

I am working with an .xdmf model using dolfinx and trying to assemble the stiffness and mass matrix associated to the Laplace equation to solve and eigenvalue problem.

When I run the code, I get the following warning: `Level 25:UFL:Expr.ufl_domain() is deprecated, please use extract_unique_domain(expr) instead.`

How should I modify my code to avoid this? I ran the same code a couple months ago and didn’t have this problem.

The lines of code I’m runing are the following:

```
with XDMFFile(MPI.COMM_WORLD, "mesh.xdmf", "r") as xdmf:
domain = xdmf.read_mesh(name="Grid")
#FEM function space
V = fem.FunctionSpace(domain, ("Lagrange", 1))
#Trial and Test functions
u = ufl.TrialFunction(V)
v = ufl.TestFunction(V)
#Stifness matrix
a = ufl.dot(ufl.grad(u),ufl.grad(v))*ufl.dx
#Mass matrix
b = u*v*ufl.dx
#Assemble
A = fem.petsc.assemble_matrix(fem.form(a))
B = fem.petsc.assemble_matrix(fem.form(b))
A.assemble()
B.assemble()
```

Thanks for any help.