Unable to unset image uniform for Material

[nmschulte-aviture]

It seems not possible to "unset" the image uniform for a Material.

Just as I can create a Material with image: undefined, I expect I can set image = undefined to achieve the same state.
However, it seems setting image = Cesium.Material.DefaultImageId ('czm_defaultImage') does not work either; nothing can achieve this, the only work-around is to create a new Material (and destroy the old one).

See the following demo (paste into Sandcastle):

var viewer = new Cesium.Viewer("cesiumContainer");
var scene = viewer.scene;
var rectangle = scene.primitives.add(
  new Cesium.Primitive({
    geometryInstances: new Cesium.GeometryInstance({
      geometry: new Cesium.RectangleGeometry({
        rectangle: Cesium.Rectangle.fromDegrees(
          -120.0,
          20.0,
          -60.0,
          40.0
        ),
        vertexFormat: Cesium.EllipsoidSurfaceAppearance.VERTEX_FORMAT,
      }),
    }),
    appearance: new Cesium.MaterialAppearance({
      flat: true,
      material: Cesium.Material.fromType(Cesium.Material.ImageType, {
        // image: '../images/Cesium_Logo_Color.jpg',
        // image: '../images/checkerboard.png',
        image: undefined,
        color: Cesium.Color.RED.withAlpha(0.5),
      }),
    }),
  })
);

Sandcastle.addToolbarButton('"Cesium_Logo_Color.jpg"', function() {
  rectangle.appearance.material.uniforms.image = '../images/Cesium_Logo_Color.jpg';
});
Sandcastle.addToolbarButton('undefined', function() {
  rectangle.appearance.material.uniforms.image = undefined;
});
Sandcastle.addToolbarButton('Cesium.Material.DefaultImageId', function() {
  rectangle.appearance.material.uniforms.image = Cesium.Material.DefaultImageId;
});
Sandcastle.addToolbarButton('"czm_defaultImage"', function() {
  rectangle.appearance.material.uniforms.image = 'czm_defaultImage';
});
Sandcastle.addToolbarButton('""', function() {
  rectangle.appearance.material.uniforms.image = '';
});
Sandcastle.addToolbarButton('"does_not_exist"', function() {
  rectangle.appearance.material.uniforms.image = 'does_not_exist';
});
Sandcastle.addToolbarButton('"checkerboard.png"', function() {
  rectangle.appearance.material.uniforms.image = '../images/checkerboard.png';
});

[dzungpng]

I can confirm that this is the current behavior. It would make more sense if setting image to undefined removes the previous image from the geometry.

I made this Sandcastle which could be a potential workaround: link. Although it also removes the base color of the geometry and setting base color to RED again does not seem to have an effect. I tried setting requestRenderMode = true and call requestRender after setting the image to undefined but that did not help.

This might be related to an older issue here: #5080