"What eh"/"how to resolve" a "null Object Reference" error? What appears on the console?

Question

When I try to use the code below, (what alias I managed to get with help from friends of the OS) an error appears on the line:
spinner.setOnItemSelectedListener(this);
With the following console code:

java.lang.Runtimeexception:
Unable to start Activity Componentinfo{ jp.co.e_grid.rakuseki/jp.co.e_grid.rakuseki.Postconfirmationactivity
}:

java.lang.Nullpointerexception:
Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null Object Reference

Here is my code:

public class JSONOAsyncTask extends AsyncTask<String, Void, Boolean> implements  AdapterView.OnItemSelectedListener{

    private Spinner spinner;
    private ArrayAdapter<String> dataAdapter;

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        Spinner spinner = (Spinner) findViewById( R.id.memo_confirmation_spinner );
        spinner.setOnItemSelectedListener(this);
    }

        @Override
        protected Boolean doInBackground(String... urls) {

            try {
                Log.e("****** MESSAGE ******", " Json Object  = " + JSONParser.getJSONFromUrl( URL ).get("ReportDetailTextList"));
            } catch (JSONException e) {
                e.printStackTrace();
            }

            // TEST: Temporaly list
            List<String> categories = new ArrayList<String>();
            categories.add("aaa");
            categories.add("bbb");
            categories.add("ccc");
            categories.add("ddd");
            categories.add("eee");

            // Adapter Creation

            // ArrayAdapter adapter = ArrayAdapter.createFromResource(getBaseContext(), R.array.confirmation_memo, android.R.layout.simple_spinner_item);
            // adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

            dataAdapter = new ArrayAdapter<String>(getBaseContext(), android.R.layout.simple_spinner_dropdown_item, categories);
            dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

            return false;
        }

    protected void onPostExecute(Boolean result) {
        // putting adapter in to data
        spinner.setAdapter(dataAdapter);

    }

    @Override
    public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

    }

    @Override
    public void onNothingSelected(AdapterView<?> parent) {

    }
}

Important detail:
When I call the same "spinner" from within the Oncreate() with that same ID, everything works fine, see code snippet:

Spinner spinner = (Spinner) findViewById(R.id.memo_confirmation_spinner);
spinner.setOnItemSelectedListener(this);

ArrayAdapter adapter = ArrayAdapter.createFromResource(this, R.array.confirmation_memo, android.R.layout.simple_spinner_item);
// Specify the layout to use when the list of choices appears
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
// Apply the adapter to the spinner
spinner.setAdapter(adapter);
// ATTENTION: This was auto-generated to implement the App Indexing API.
// See https://g.co/AppIndexing/AndroidStudio for more information.

I have read other posts about "null Object Reference" in the SO but it seems to me that there are many variations of the same error, so here is my question: What this error message refers to and, how to resolve it?

Answer 1

Briefly the exception Nullpointerexception is launched whenever you try to access a memory object that has not been instantiated, or better initialized, until the moment of your call.

First factor to be noted, is that you are declaring two variables of the type Spinner, globally in its Inner class and inside onPreExecute(). Declare only once so that no error occurs. See how your method would look onPreExecute():

@Override
protected void onPreExecute() {
    super.onPreExecute();
    spinner = (Spinner) findViewById(R.id.memo_confirmation_spinner );
    spinner.setOnItemSelectedListener(this);
}

Second factor is that as you are saying that the error is in this line below:

spinner = (Spinner) findViewById( R.id.memo_confirmation_spinner );

Try to observe in your out class if you’re not calling your JSONOAsyncTask before the setContentView(). If so, just reverse doing so:

setContentView(R.layout.sua_activity);
new JSONOAsyncTask.execute();