Getting "Some referenced Items could not be found" error in process model

Hello,

Whenever I'm launching a task there is an option to assign it to some particular groups. If no group is being selected then in that case it will be assigned to default group. It was working fine since yesterday but I made some changes related to the assignment. Now what happening is if I will give the value to assign any particular group, It works fine but if i will leave it blank then in that case it gives "Some referenced item could not be found error". Can anyone please tell me why is this happening?

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