graph: "Ungroup this series of nodes" button is broken

[psybuzz]

Repro:

• Open the Graph dashboard
• Load a graph that contains a collapsible series of nodes, e.g. series_graph.txt
• Select the series group, and click "Ungroup this series of nodes" button in the panel
  

Expected: it should expand the series group

Actual: Nothing happens, and the console shows an error

Note that double-clicking the grouped series node to expand it (different from ungrouping it) still works.

[psybuzz]

I manually checked with tensorboard==2.2.2 (broken), and tensorboard==1.14 (good).