Null checks for MultiMap methods

I used NullPointerException because IMap already uses that exception when a user passes null value as parameter. However I'm not sure whether it should be IllegalArgumentException or not.
Do you think that creating a method ```ValidationUtil.testIsNull(String parameter, String message)``` that throws NullPointerException is a good practice? We can also use that method in other proxy classes.
It seems there is also another method ```ValidationUtil.isNotNull(E argument, String argName)``` but it throws IllegalArgumentException so I'm not sure about the name of the method that we will create.