Stacker: can't add new segment to stacked SED

[jbudynk]

After stacking a group of SEDs, the resultant SED is added to the SED Builder. A silent java.lang.NullPointerException exception is raised when a user tries to add new segments to this SED. No warning pops up to the user.

[jbudynk]

Note that this was in the list of known caveats on the webpage.