Crash in progress dialog monitor: #4119
Crash in progress dialog monitor: #4119

This has proven to be difficult to reproduce, but I believe this patch fixes a problem where
the progress dialog might be destroyed while inside its own monitor callback.

ritchiewilson/shotwell,h4ck3rm1k3/shotwell,jof/shotwell,jimbonator/shotwell,Sanva/shotwell-gsoc,jimbonator/shotwell,damianmoore/shotwell,polymeris/shotwell,damianmoore/shotwell,abrauchli/shotwell,abrauchli/shotwell,jimbonator/shotwell,polymeris/shotwell,ritchiewilson/shotwell,Sanva/shotwell-gsoc,h4ck3rm1k3/shotwell,ritchiewilson/shotwell,damianmoore/shotwell,Sanva/shotwell-gsoc,h4ck3rm1k3/shotwell,jof/shotwell,jof/shotwell,polymeris/shotwell,abrauchli/shotwell