"An unknown error occured" in groovy while accessing substitution variables

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Hi guys!

There is a small issue. I am trying to access substitution variable which contains empty,Freeze,Freeze,Freeze as highlighted in the below image.

For this, I have used operation.application.getSubstitutionVariable('Sub_freeze_unfreeze').getValue() 

and then used split(",") to store it as an array in a variable current_freeze_unfreeze.

So, I am getting the expected output which is [empty,Freeze,Freeze,Freeze]

Now, I need to change the array to [empty,Freeze,Unfreeze,Freeze] .

But, when I am using = , An error is being encountered as below.

Line 60 is highlighted in the full code below.