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

data/graph.js

@@ -267,7 +267,9 @@ function initGraph() {
     });
     var endDraw = Date.now();
     draws.push(endDraw - lastTick);
-    nodes.call(force.drag);
+    if (force) {
+      nodes.call(force.drag);
+    }
 
     ticking = false;
   });