How to use GeckoSessionSettings correctly

I am totally new to Java but tried to make a really simple app which displays a website inside a GeckoView.

I could archive to load a website inside a GeckoView but once I try to change some Settings (UserAgentMode to DESKTOP) the app crashed once I try to open it on the phone (no errors shown). As I said I am totally new to Java - so it might be a beginner mistake - anyhow I can't figure it out on my own.

Here is the documentation for UserAgentMode: https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/GeckoSessionSettings.html#USER_AGENT_MODE_DESKTOP

Where I am at the moment:

package com.example.geckotest;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;

import org.mozilla.geckoview.GeckoRuntime;
import org.mozilla.geckoview.GeckoSession;
import org.mozilla.geckoview.GeckoSessionSettings;
import org.mozilla.geckoview.GeckoView;

public class MainActivity extends AppCompatActivity {

    private GeckoView geckoview;
    private GeckoSession GeckoSession;
    private GeckoRuntime geckoRuntime;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        GeckoView view = findViewById(R.id.geckoview);
        GeckoSession session = new GeckoSession();

        GeckoSession.getSettings().setInt(GeckoSessionSettings.USER_AGENT_MODE, 1);

        GeckoRuntime runtime = GeckoRuntime.create(this);


        session.open(runtime);
        view.setSession(session);
        session.loadUri("https://google.com");

    }
}

This line causes the problem; when I comment them out, the app starts just fine - but not in the desired desktop mode:

GeckoSession.getSettings().setInt(GeckoSessionSettings.USER_AGENT_MODE, 1);