On line 377 of FBXUnityMeshGetter.cs, must add a null check before getting the reference. If a null material is provided the system is unable to export.
Material mat = allMaterialsInThisMesh[i];
int referenceId = Mathf.Abs(mat.GetInstanceID());
if(mat == null)
...
On line 377 of FBXUnityMeshGetter.cs, must add a null check before getting the reference. If a null material is provided the system is unable to export.