How update an Activity on "onResume"?

Question

I have a listview filled with some text. But when the listview is empty, it will show a button that takes you to a new activity, where you type some text and it saves on SharedPreferences and finishes. I want the Listview's Activity to update when the other one is closed. I tried onResume() but for some reason it stays refreshing forever. How do I do that?
By the way, I'm updating using:
startActivity(getIntent()); finish();
Is there a better way to do it?
Thanks.

EDIT-----------------------------------------------
As @Alex Loper said, I added notifyDataSetOnChange() in the onResume() method. But it crashed with this log:

12-31 19:45:05.284  11864-11864/com.nerv.cas.partying E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.nerv.cas.partying, PID: 11864
java.lang.RuntimeException: Unable to resume activity {com.nerv.cas.partying/com.nerv.cas.partying.YourPartiesActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2812)
        at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2845)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2262)
        at android.app.ActivityThread.access$800(ActivityThread.java:139)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5086)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at com.nerv.cas.partying.YourPartiesActivity.onResume(YourPartiesActivity.java:48)
        at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1215)
        at android.app.Activity.performResume(Activity.java:5327)
        at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2797)
            at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2845)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2262)
            at android.app.ActivityThread.access$800(ActivityThread.java:139)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5086)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)

My class is like this:

ArrayAdapter<String> adapter;
User user;

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

    user = getUserData();
    fillUserParty(user);
    fillAdapter(user);

    ListView partiesListView = (ListView) findViewById(R.id.parties_list_view);
    Button goToCreatePartyButton = (Button) findViewById(R.id.go_to_create_party_button);
    if(adapter != null){
        partiesListView.setVisibility(View.VISIBLE);
        partiesListView.setAdapter(adapter);
    }else{
        goToCreatePartyButton.setVisibility(View.VISIBLE);
        goToCreatePartyButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(YourPartiesActivity.this, CreatePartyActivity.class));
            }
        });
    }
}

@Override
protected void onResume() {
    super.onResume();
    adapter.notifyDataSetChanged();
}

private void fillAdapter(User user){
    List<Party> parties = user.getParties();
    if(parties != null) {
        for (int i = 0; i < parties.size(); i++) {
            Party party = parties.get(i);
            adapter.add(party.getName());
        }
    }
}

private void fillUserParty(User user){
    SharedPreferences preferences = getSharedPreferences("Parties", MODE_PRIVATE);
    int numOfParties = CreatePartyActivity.checkNumberOfParties(preferences);
    for(int i=0; i<numOfParties; i++){
        String partyName = preferences.getString("Name" + i, "");
        if(partyName != ""){
            int id = preferences.getInt("Id" + i, 0);
            Party party = new Party(id, partyName);
            user.joinParty(party);
        }
    }
}

public User getUserData(){
    SharedPreferences preferences = getSharedPreferences("User", 0);
    int id = preferences.getInt("Id", 0);
    String username = preferences.getString("Username", "");
    String name = preferences.getString("Name", "");
    return new User(id, username, name);
}

Answer 1

I can't find initialize adapter in posted code. I think adapter always is null!

To resolve null pointer exception, change onResume method like this :

@Override
protected void onResume() {
  super.onResume();
  if (adapter != null)
     adapter.notifyDataSetChanged();
}