Fix 'NoneType' object has no attribute 'name' on export

When a team is deleted, its image sets are kept, but the team is changed
to None. When the annotations of the set are exported, the team name is
accessed which results in the above mentioned error. Now, the team name
will be replaced with an empty string if the team is None.

Author: timonegk

imagetagger/imagetagger/annotations/views.py

@@ -234,11 +234,11 @@ def export_format(export_format_name, imageset):
 
     placeholders_filename = {
         '%%imageset': imageset.name,
-        '%%team': imageset.team.name,
+        '%%team': imageset.team.name if imageset.team else 'DELETED',
         '%%setlocation': imageset.location,
     }
     for key, value in placeholders_filename.items():
-                        file_name = file_name.replace(key, str(value))
+        file_name = file_name.replace(key, str(value))
 
     min_verifications = export_format.min_verifications
     annotation_counter = 0
@@ -422,7 +422,7 @@ def export_format(export_format_name, imageset):
         '%%content': formatted_content,
         '%%imageset': imageset.name,
         '%%setdescription': imageset.description,
-        '%%team': imageset.team.name,
+        '%%team': imageset.team.name if imageset.team else 'DELETED',
         '%%setlocation': imageset.location,
     }
     for key, value in placeholders_base.items():