server crash from closing a notebook from the start page while it is still open

[scottdraves]

if you open a notebook, and then open the start page in another tab, and then click the "close" button, then go back to the tab where the notebook is still open, and then edit or save it, the server crashes:

java.lang.NullPointerException
        at com.twosigma.beaker.core.rest.SessionBackupRest.setEdited(SessionBackupRest.java:223)

instead you should get a pop-up notification "this session has been closed from another window" similar to #4308