fix(layer_dialog): make layer dialog robust to findSourceGroup errors
fix(layer_dialog): make layer dialog robust to findSourceGroup errors

google/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer,google/neuroglancer