Fix Issues in neuroglancer-with-segments notebook when widget() is used.

[tkakar]

When executing the widget_neuroglancer_precomputed_with_segments notebook, the following errors occur only if vc.widget() is used for visualization. vc.web_app() works fine.

• 'obsColorEncoding': 'obsColors' throws the following errors related to worker threads.
• when above is commented, the meshes load but the loading overlay does not go away.

0c482b40-4ca0-4373-bdd5-cf5107cff564:10137 Uncaught TypeError: Cannot convert undefined or null to object
    at Object.assign (<anonymous>)
    at RPC.<anonymous> (0c482b40-4ca0-4373-bdd5-cf5107cff564:10137:14)
    at RPC.target.onmessage (0c482b40-4ca0-4373-bdd5-cf5107cff564:5583:43)

0c482b40-4ca0-4373-bdd5-cf5107cff564:13515 Uncaught TypeError: Cannot read properties of undefined (reading 'add')
    at onTemporaryVisibleSegmentsStateChanged (0c482b40-4ca0-4373-bdd5-cf5107cff564:13515:78)
    at new SegmentationLayerState (0c482b40-4ca0-4373-bdd5-cf5107cff564:13570:9)
    at new MultiscaleMeshLayer (0c482b40-4ca0-4373-bdd5-cf5107cff564:14666:9)
    at RPC.<anonymous> (0c482b40-4ca0-4373-bdd5-cf5107cff564:5738:17)
    at RPC.target.onmessage (0c482b40-4ca0-4373-bdd5-cf5107cff564:5583:43)

onTemporaryVisibleSegmentsStateChanged @ 0c482b40-4ca0-4373-bdd5-cf5107cff564:13512
SegmentationLayerState @ 0c482b40-4ca0-4373-bdd5-cf5107cff564:13570
MultiscaleMeshLayer @ 0c482b40-4ca0-4373-bdd5-cf5107cff564:14666
(anonymous) @ 0c482b40-4ca0-4373-bdd5-cf5107cff564:5738
RPC.target.onmessage @ 0c482b40-4ca0-4373-bdd5-cf5107cff564:5583
Worker.postMessage
invoke @ ReactNeuroglancer-BfWBfio7.js:5646