Kernel feels stuck with multiple execution of cells in Neuroglancer-view notebooks

[tkakar]

Any time a neuroglancer containing view is generated using the widget, the kernel seems stuck with loading overlay not going away.

This some times gets resolved by shutting down all kernels. Stopping all servers using vitessce.data_server.stop_all() did not work.

Below are the errors seen in the console

87aba817-cd67-460f-86a8-9c50cbf03dfb:13575 [TMT] SegmentationLayerState construct #1 Object
87aba817-cd67-460f-86a8-9c50cbf03dfb:13589 [TMT] construct #1: visibleSegments is undefined -- options.visibleSegments was 11 (a real id, but rpc2.get() returned undefined -- a real registration-order race)
SegmentationLayerState @ 87aba817-cd67-460f-86a8-9c50cbf03dfb:13589
87aba817-cd67-460f-86a8-9c50cbf03dfb:13589 [TMT] construct #1: segmentEquivalences is undefined -- options.segmentEquivalences was 12 (a real id, but rpc2.get() returned undefined -- a real registration-order race)
SegmentationLayerState @ 87aba817-cd67-460f-86a8-9c50cbf03dfb:13589
87aba817-cd67-460f-86a8-9c50cbf03dfb:13589 [TMT] construct #1: temporaryVisibleSegments is undefined -- options.temporaryVisibleSegments was 13 (a real id, but rpc2.get() returned undefined -- a real registration-order race)
SegmentationLayerState @ 87aba817-cd67-460f-86a8-9c50cbf03dfb:13589
87aba817-cd67-460f-86a8-9c50cbf03dfb:13589 [TMT] construct #1: temporarySegmentEquivalences is undefined -- options.temporarySegmentEquivalences was 14 (a real id, but rpc2.get() returned undefined -- a real registration-order race)
SegmentationLayerState @ 87aba817-cd67-460f-86a8-9c50cbf03dfb:13589
87aba817-cd67-460f-86a8-9c50cbf03dfb:13589 [TMT] construct #1: useTemporaryVisibleSegments is undefined -- options.useTemporaryVisibleSegments was 15 (a real id, but rpc2.get() returned undefined -- a real registration-order race)
SegmentationLayerState @ 87aba817-cd67-460f-86a8-9c50cbf03dfb:13589
87aba817-cd67-460f-86a8-9c50cbf03dfb:13589 [TMT] construct #1: useTemporarySegmentEquivalences is undefined -- options.useTemporarySegmentEquivalences was 16 (a real id, but rpc2.get() 

Related to #517