How to make "if(String is found on locally){...}

I want to make an int statement such that if there is a string saved on the local database, then do whatever. I tried:

public class DispatchActivity extends Activity {
    SharedPreferences sharedPref;
    SharedPreferences.Editor editor;
    @Override
    public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        sharedPref= getSharedPreferences("myPref", Context.MODE_PRIVATE);
        editor=sharedPref.edit();
        String userIDsaved = sharedPref.getString("user_id", "");
    if(userIDsaved==null){
        Intent startSignUpActivity = new Intent(DispatchActivity.this,SignUpActivity.class);
        DispatchActivity.this.startActivity(startSignUpActivity);
    }
        else{
        Intent startMainActivity = new Intent(DispatchActivity.this,MainActivity.class);
        DispatchActivity.this.startActivity(startMainActivity);
    }
...

However the app crashes when I do that. This is a dispatch activity, meaning if there is a string found locally, then start the mainactivity. if not, start the sign up activity. Even If i do have a string saved locally, then it crashes. Please tell me how to do what I mentioned