Bug 435385: [RAP] Widget disposed exception in the date field within an
editable table.

https://bugs.eclipse.org/bugs/show_bug.cgi?id=435385
Since the focusControl comes from an member variable, it may be disposed
in the meanwhile. Therefore we need to add a check. Backport of commit
c05ef276385e51eec5d49000a45b3f40fb7984e9

Change-Id: I665f6cb65552b67eeda2549f0cf683e2aafa9591
Reviewed-on: https://git.eclipse.org/r/38529
Tested-by: Hudson CI
Reviewed-by: Matthias Nick <Matthias.Nick@bsiag.com>
Tested-by: Matthias Nick <Matthias.Nick@bsiag.com>