Cannot delete priority of tasks once one has been added

[00100200345]

Add a task, add priority, save task. Then re-open and try to change priority to "none" - error.

[felixevers]

Variable '$data' got invalid value 'none' at 'data.priority'; Value 'none' does not exist in 'TaskPriority' enum.

[felixevers]

Reproducable.