backported fix for # 11694

http://hub.qgis.org/projects/quantum-gis/repository/revisions/810cb3fb999d9fdd6c1d6fe7b353ab83ce85b17c/diff/python/plugins/processing/modeler/ModelerParametersDialog.py
Merge pull request #1723 from anitagraser/patch-5

backported fix for # 11694