Return an empty DocumentPage Queryset when a document has no latest version.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/documents/models.py

@@ -319,7 +319,7 @@ class Document(models.Model):
             return self.latest_version.pages
         except AttributeError:
             # Document has no version yet
-            return 0
+            return DocumentPage.objects.none()
 
 
 class DeletedDocument(Document):

mayan/apps/documents/widgets.py

@@ -285,7 +285,7 @@ class DocumentThumbnailWidget(BaseDocumentThumbnailWidget):
         return getattr(instance, 'label', None)
 
     def is_valid(self, instance):
-        return instance.latest_version.pages.all()
+        return instance.pages
 
 
 class DocumentPageThumbnailWidget(BaseDocumentThumbnailWidget):