There are 2 types of attributes
"workflowname" for storing the workflow named "workflowname" when editing: but this could lead to conflict if the workflowname is the same as an existing used session attribute (such as the logged user)
"WORKFLOW_NAMES" holding the list of all workflows

we should have a single attribute holding a map or a list or a set of the worflows
(avoid conflicts and avoid inconsistency between all thoses attributes)

+

private static final String __WORKFLOW_NAMES = "workflowNames";
should be something like
WorkflowSessionHelper.class.getName() + "$workflowNames";