The title says it all. It appears that if you leave the "location of time field" blank, Mango throws a NullPointerException. It seems the other required values there tell you that they are required, rather than tossing NullPointerException.
Next time you go through updating the Excel Reports module, maybe have a quick look at the error handling to get the "location of time field" caught if it's blank.
This is in Mango 3.6.5