Running the solver on a slight modified example from https://github.com/cb-geo/mpm-benchmarks/tree/develop/3d/hydrostatic_column gives NaN vtp output #749

{
  "title": "Hydrostatic column",
  "mesh": {
    "mesh": "mesh-1.txt",
    "entity_sets": "entity_sets-1.json",
    "check_duplicates": true,
    "cell_type": "ED3H8",
    "isoparametric": false,
    "io_type": "Ascii3D",
    "node_type": "N3D",
    "boundary_conditions": {
        "velocity_constraints": [
            {
                "nset_id" : 0,
                "dir": 0,
                "velocity": 0.00
            },
            {
                "nset_id" : 1,
                "dir": 1,
                "velocity": 0.00
            },
            {
                "nset_id" : 2,
                "dir": 2,
                "velocity": 0.00
            }
        ]
    }
  },
  "particles": [
    {
      "generator": {
        "check_duplicates": true,
        "particle_type": "P3D",
        "cset_id": -1,
        "pset_id": 0,
        "material_id": 1,
        "nparticles_per_dir": 2,
        "type": "gauss"
      }
    }
  ],
  "materials": [
    {
      "id": 0,
      "type": "Newtonian3D",
      "density": 1800,
      "bulk_modulus": 1.0E+06,
      "dynamic_viscosity": 0
    },
    {
      "id": 1,
      "type": "LinearElastic3D",
      "density": 1800,
      "youngs_modulus": 1.0E+06,
      "poisson_ratio": 0
    }
  ],
  "external_loading_conditions": {
      "gravity": [0, 0, -9.81]
  },
  "analysis": {
    "type": "MPMExplicit3D",
    "stress_update": "usl",
    "dt": 0.00001,
    "uuid": "hydrostatic_usl",
    "nsteps": 10000,
    "velocity_update": true,
    "resume": {
      "resume": false,
      "uuid": "hydrostatic_usl",
      "step": 7000
    }
  },
  "post_processing": {
    "path": "results1/",
    "output_steps": 1000,
    "vtk": [],
    "vtk_statevars":[
   {
   	"phase_id":0,
    	"statevars":[
    		"displacements",
    		"velocities",
    		"stresses",
    		"strains"
    	]
    },
    
    {
   	"phase_id":1,
    	"statevars":[
    		"displacements",
    		"velocities",
    		"stresses",
    		"strains"
    	]
    }
    ]
  }
}