VUGEN 11.52 wont stop

Hi, Im currently using VUGEN 11.52. I've been trying to enhance a certain script but troubleshooting is almost impossible because I cannot stop once i run a script. Even if vusers have been terminated still vugen is in a running state.. i ve been getting this messages before and while running a script.

Source: mscorlib

Target method: System.Object CreateInstance(System.RuntimeType, Boolean, Boolean, Boolean ByRef, System.RuntimeMethodHandleInternal ByRef, Boolean ByRef)

Message: Index was out of range. Must be non-negative and less than the size of the collection.Parameter name: index

Source: HP.LR.VuGen.StepsNavigator
Target Method: Void RefreshCollectionView()
Message: Object reference not set to an instance of an object.

I can only stop the script by using the task manager.

Thanks in advance.