Work Flow error in Testplan module- Object reference not set to an Instance of an object

Hello , 

Can any one  help me in resolving the error in Test Plan Module.

Work flow error in Function Set Environment List  -  "Object reference not set to an Instance of an object"  

Am creating new TestPlan workflow as per the customer request , i implemented the below script 

Sub Test_SetLists_Environment()
Dim listName
On Error Resume Next

If Test_Fields("TS_STATUS").Value = ""  then
listName = "NewValue"

ElseIf Test_Fields("TS_STATUS").Value = "New" then
listName = "TCW-10-010"
ElseIf Test_Fields("TS_STATUS").Value = "In Progress" then
listName = "TCW-10-020"
ElseIf Test_Fields("TS_STATUS").Value = "Completed" then
listName = "TCW-10-030"
ElseIf Test_Fields("TS_STATUS").Value = "Rebuit" then
listName = "TCW-10-040"
Test_Fields("TS_STATUS").IsReadOnly = True
End If

Test_Fields("TS_STATUS").List = Lists(listName)
PrintError ("Set Environment List")

On Error GoTo 0