Hello Andy,
Can you please check, if Export Depth works in the version 1.4.2 build 6205 properly?
Actually, Export Depth operation is not using depth maps generated during the dense cloud calculation, the depth is estimated as a distance from the camera to the mesh model and would work even for unscaled models.
So to complete this task you need to have mesh model and Null Model error indicates that there's no mesh in the active chunk.