Added undefined check to tracepoints box update method

The updae method in the tracepoints box view uses jQuery's $.map method
which doesn't check for null references and fails loudly when there are
no selected tracepoints in the tracepoints API.

I haved added a null check that fixes the issue.

Fixes #53.
Merge pull request #55 from dzautner/fix-tracepoint-bug

Added undefined check to tracepoints box update method