Object instance 1200 does not exist (while executing task SWUS)

hello all,

i have created a workflow class and two methods (constructor, display) and 1 attribute PLANT (instance,public) in it.

i am using this class and display method in task t code PFTC.

While executing this task from SWUS , i am getting this error.Object instance 1000 does not exist.

As i was going through this blog. i have created everything exactly just like it is mentioned there.

But still i am getting this error. please guide me to detect where i am mistaken.

http://scn.sap.com/community/bpm/business-workflow/blog/2006/07/25/using-abap-oo-methods-in-workflow-tasks

this is class screen-shot and error i am facing while executing the task.

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Image may be NSFW.
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The methods which are implemented using the interface are emtpy(don't contain any source code lines).