examples/advection-reaction-dg.cc: no longer builds with DUNE 2.4.1

examples/advection-reaction-dg.cc in the releases/2.4-compatible branch no longer builds with DUNE 2.4.1:

[...]/dune-functions/examples/advection-reaction-dg.cc:492:103:   required from here
[...]/dune/grid/io/file/vtk/vtkwriter.hh:269:53: error: invalid initialization of reference of type [...]

advection-reaction-dg.cc:492 is

  vtkWriter.addVertexData(vtkFunction(xFunction), VTK::FieldInfo("x", VTK::FieldInfo::Type::scalar, 1));

which uses the vtkFunction adapter for DUNE 2.4.