Fenics solver and user-defined functions

I am trying to solve using the Fenics solver the heat transfer equation considering temperature-dependent parameters. However, I am getting the following error:



It seems that the user-defined function density  needs to be included somehow in the solver. Any thoughts on how this can be fixed? I am using FEATool Multiphysics as a Matlab toolbox on Linux.

Hard to say why the density was not defined without more information, but the workaround would be to manually edit the converted Python code (using the "Script" tab in the "FEniCS" solver settings dialog box) and add the variable manually before calling the solver.