[FIX] Fixed Exception in ProgressActivity

We had a ProgressActivity without a target which - after commit 98a8e89
adding a parameter-test for "target != null" - resulted in an IAE. The
Activity in question was used as a Poison Pill to the loop in which a
ProgressMonitor was updated and is therefore never serialized.

Change-Id: I7f6c464a4111ecfbefb7df668823b0beda2f91d6
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/868
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Patrick Schlott <pschlottk@gmail.com>