Unexpected runtime error

The runtime has encountered an unexpected error.

Error source

Page Name:/hisHomePage.xsp
Control Id: viewColumn5
Property: iconSrc

Exception

Error while executing JavaScript computed expression
Script interpreter error, line=2, col=8: 'doc' is null

Javascript code

   1: var doc = col.getDocument()
   2: if(doc.getItemValueString("PhotoUrlThumb")==""){
   3: return "photo_none.gif"
   4: 	}
   5: else{
   6: 	
   7: return  doc.getItemValueString("PhotoUrlThumb")
   8: }

► Stack Trace