The question isn't really clear, so I will attempt a general answer.
When you declare a global variable which is an object (array, String, ArrayList, etc. or a class you created), it is initialized by default to null, as it is not valid yet: the variable actually holds a reference to an object, null denotes no reference at all, the variable is "empty".
If you try to call a method on such variable, you would get a NullPointerException, as there is no object (class instance) behind it (referenced) to execute the method. That's why sometime we test if the variable is null before using it. For example some methods return an object, or null to indicate an error.