How to get value from Dropdown to in string in script runner

[Pravin Awati]

I am adding this code to post function of workflow 

i need to to compare the value which is selected by user from Dropdown i tried below code 

CustomFieldManager customFieldManager108 = ComponentAccessor.getCustomFieldManager()

def tType = customFieldManager108.getCustomFieldObjectByName("Function Providing the Approval")

def fieldVal = issue.getCustomFieldValue(tType).toString()

but unfortunately i am getting null value in fieldVal  

can you please help on this