Mock Class within an Class- Using the original behavior of these Classes when calling methods

I am having an issue with Null Pointer while trying to use Mockito with UT

ServiceClass{
    @Inject
    private UtilsOne;
...
    public void methodOne(object1, object2, List<> elements){
       --- some logic ---
       UtilsOne.method1();
       --- some logic ---
       UtilsOne.method2();
       --- some logic ---
       Element e = UtilsOne.createElement(type,id);
       elements.add(e);
    }
...
}

UtilsOne{
    @Inject
    private UtilsTwo;
...
    public void method1(){
       --- some logic ---
    }
    public void method2(){
       --- some logic ---
    }
    public Element createElement(Object type, String id){
       --- some logic ---
       UtilsTwo.callMethod(type, id);
    }
...
} 

This is how the structure of the classes looks in general.

I used @InjectMocks on the ServiceClass and @Spy on the UtilsOne so far the functionality worked for me, when calling methodOne, everything seems to work as expected but,

inside methodOne when reaching the UtilsTwo.callMethod() it throw a Null Pointer exception, I tried to figure out how to make the class (UtilsTwo) not Null so it will work but I did not figure it out.

If something is not clear please tell me I will edit and explain it better Thanks in advance.

***Edited

After reading the answer I made it clearer, I want to check the content of the List<> elements, each Utils is has its own method and they have a connection - it will not be right to change the structure as it is now

After a lot trial and error, I used ReflectionTestUtils and it solved my problems

This is the general structure ReflectionTestUtils.setField(ClassWhereTheFieldIsAt, "NameOfFieldInClass", NameOfFieldInTest);

in the test class if it looks like this

@Spy 
private UtilsOne utilsOne;

private UtilsTwo utilsTwo;
@Test 
public void testElements(){
   ReflectionTestUtils.setField(utilsOne, "utilsTwo", utilsTwo);

....
}

For me the the NameOfFieldInClass and NameOfFieldInTest are the same.