When a string property was set to None, it was converting itself to a string 'None'.

fsl/props/properties.py

@@ -116,7 +116,6 @@
 
 import logging
 
-from collections import OrderedDict
 
 log = logging.getLogger(__name__)
 

fsl/props/properties_types.py

@@ -139,8 +139,10 @@ class String(props.PropertyBase):
 
 
     def cast(self, value):
+
+        if value is None: value = ''
+        else:             value = str(value)
         
-        value = str(value)
         if value == '': value = None
         return value