Why in a variable density case the density should be set?

Hi, everyone. I follow the example of mfix-19.3.0\legacy_tutorials\variable_density to set in GUI. The usr_rates.f is pasted from the variable_density as well. When I run mfix, something goes wrong, like this:image image image
Here is the case:2D TFM fluid bed - modified.zip (29.0 MB)
And the bug report:2d_tfm_modified_2020-03-10T113321.885425.zip (34.1 KB)
It is interesting that if I set the biomass density is constant errors will disappear. But this case is to simulate the variable biomass density during the reaction. So how to deal with these problems?
Best regards