Updating dof of dolfinx.fem.function.Function using Midpoint time stepping

General
2

Referencing https://fenicsproject.discourse.group/t/dolfinx-fem-function-function-add-with-dolfinx-fem-function-function/9328

I replaced

Ch = C_n + dt/g_mid*(f- div(phi_mid))

with

expr = dolfinx.fem.Expression(C_n + dt/g_mid*(f- div(phi_mid)),V.element.interpolation_points())
Ch.interpolate(expr)

Is this on the right track?