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I'm using a plugin in my application, everything is done. But when i check the log, there are plenty exception as shown below. 
The good thing is, it doesn't influence any result there, but the bad thing is as it doesn't ruin anything, I could not tell where it takes place and what caused that strange stuff.
In my plugins, I've used the many stuff as "option = AppUtil.processHashVariable(option, wfAssignment, null, null);" to get the value by hash variable.
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