Fix broken graph update (fix #586)

This addresses the following console error when switching from the
graph tab to the list tab:

    System JS : ERROR resource://gre/modules/XPIProvider.jsm
     -> jar:file:///tmp/tmpEhX8Bu.mozrunner/extensions/jid1-F9UJ2thwoAm5gQ@jetpack.xpi!/bootstrap.js
     -> resource://gre/modules/commonjs/toolkit/loader.js
     -> resource://gre/modules/commonjs/sdk/loader/sandbox.js
     -> resource://jid1-f9uj2thwoam5gq-at-jetpack/lightbeam/data/graph.js:270 - TypeError: force is null