Fix null exception in DockContentHandler.
Fix null exception in DockContentHandler.

This happens when putty closes and then it tries to focus on the
current Pane. The pane no longer exists, so we need to put in a check
for it.

phendryx/superputty,primeinc/superputty,primeinc/superputty,den-efremov/superputty,misodengaku/superputty,misodengaku/superputty,den-efremov/superputty,misodengaku/superputty,primeinc/superputty,den-efremov/superputty,den-efremov/superputty