Image Gradient filter giving warnings when no active scalars

When you apply the Gradient filter (vtkImageGradient under the covers), you get thousands of the following warning if your data does not have an active scalar field (and instead selected from a list).

ERROR: In C:\bbd\2d618e80\build\superbuild\paraview\src\VTK\Common\DataModel\vtkImageData.cxx, line 1324
vtkImageData (000002942B446150): No Scalar Field has been specified - assuming 1 component!

To replicate, do the following.

1. Open disk_out_ref.ex2. Load all variables. Apply.
2. Add Resample To Image filter. Apply.
3. Add the Gradient filter. Apply.

The output messages window will pop up with lots and lots of the aforementioned warning.

This problem was reported on discourse