Can't access variable from main project?

I cannot access the hi variable from my library class. Why? Check it out:

I have this inteface in my library:

interface ContextAccessor {

    fun getApplicationContext(): Application?
}

And this code as well:

class SomeLibraryClass {
    private var mContextAccessor: ContextAccessor?

    String extractedHi = null

    fun setContextAccessor(contextAccessor: ContextAccessor?) {
        mContextAccessor = contextAccessor
    }
    
    fun someOtherMethod() {
        mContextAccessor?.getAppContext()?.let { nonNullContext ->
            // use nonNullContext here
            extractedHi = nonNullContext.hi; // i get error here!
        }
    }
}

And this class in my project:

public class MyActivity extends Activity implements  MyActivity.ContextAccessor {
    
    private SomeLibraryClass someLibraryClassInstance = SomeLibraryClass();

    public String hi = "hi";

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // ContextAccessor reference is set to some library class
        someLibraryClassInstance.setContextAccessor(this);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // Super important!
        someLibraryClassInstance.setContextAccessor(null);
        // OR create some method like `someLibraryClassInstance.removeContextAccessor(this)`
    }

    @Override
    public Application getApplicationContext() {
        return super.getApplication();
    }
}

Add hi property to the ContextAccessor interface:

interface ContextAccessor {

    val hi: String
    // ...
}

And in MyActivity implement getHi() method:

@NotNull
@Override
public String getHi() {
    return hi;
}

In your library class SomeLibraryClass you can access to it like the following:

var extractedHi: String? = null

fun someOtherMethod() {
    extractedHi = mContextAccessor?.hi
}