Noob Object Help Please

Basically, I want to have a field inside an object, that optionally points to another object. If the object that is being pointed to is null, I need to be able to respond to that in code. In this example code, I set up 2 objects, they have the same fields. ObjectB points to ObjectA via one of the fields, however when ObjectA becomes nullified, I can't find a way to verify that in code? Thanks for any insight you can offer. ;)

Type testobj
 Field x:Int
 Field obj:testobj
End Type
'
Global mylist:TList = New TList
'
Global objA:testobj = New testobj
objA.x=Rand(100,800)
ListAddLast(mylist,obja)
'
Global objB:testobj = New testobj
objB.x=Rand(100,800)
objB.obj = objA   ' add a reference to obja 
ListAddLast(mylist,objb)
'
Print "Items in list:" + CountList(mylist) 
'
ListRemove(mylist,objA)  ' remove item from list
'
Print "Items in list:" + CountList(mylist) 
'
Print "After removing object A from the list..."
'
If objA = Null          
 Print "object A is null"
Else
 Print "object A is still alive"
EndIf
'
objA = Null
'
Print "After setting object A to null..."
'
If objA = Null          
 Print "object A is null"
Else
 Print "object A is still alive"
EndIf
'
' I need to find out if objB.obj is pointing to a null object!
'
' Try 1....?
If objB.obj = Null
 Print "object B reference to A is null"
Else
 Print "object B reference to A is NOT null"
EndIf
' Try 2....?
If testobj(objB.obj) = Null
 Print "obeject B reference to A is null"
Else
 Print "object B reference to A is NOT null"
EndIf