Fix for export of <Case> when condition is None...

Author: pgleeson

lems/model/dynamics.py

@@ -185,11 +185,12 @@ def toxml(self):
         """
         Exports this object into a LEMS XML object
         """
-
-        return (
-            '<Case condition="{0}" value="{1}"'.format(self.condition, self.value)
-            + "/>"
+        cond = (
+            ' condition="{0}"'.format(self.condition)
+            if self.condition is not None
+            else ""
         )
+        return '<Case{0} value="{1}"'.format(cond, self.value) + "/>"
 
 
 class ConditionalDerivedVariable(LEMSBase):