REG->6.10: widgetgraphgallery produces warnings flood "Vertex buffer empty"

Steps to reproduce:

• Launch examples/graphs/3d/widgetgraphgallery
• In the bar graph, switch to "Model Proxy Data"
• Note warnings

  Mesh building failed: Vertex buffer empty
  .....
  Mesh building failed: Vertex buffer empty
  Mesh building failed: Vertex buffer empty
  Mesh building failed: Vertex buffer empty
  Mesh building failed: Vertex buffer empty
  Mesh building failed: Vertex buffer empty
  QRhi 0x760c20002b30 going down with 4 unreleased resources that own native graphics objects. This is not nice.
    RenderBuffer resource 0x16712630 ()
    TextureRenderTarget resource 0x14a1d370 (QQuick3DDirectionalLight shadow map)
    Texture resource 0x760bf8004440 ()