Fix error where displaying Date or Confidence in Citation Tree View gave AttributeError: 'NoneType' object has no attribute 'encode'

svn: r23062

gramps/gui/views/treemodels/treebasemodel.py

@@ -976,7 +976,7 @@ class TreeBaseModel(GObject.GObject, Gtk.TreeModel):
         if not secondary:
             # None is used to indicate this column has no data
             if self.fmap[col] is None:
-                return None
+                return ''
             value = self.fmap[col](data)
         else:
             if self.fmap2[col] is None: