How to solve a problem with loading spinner items?

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Android

<pre>01-21 02:45:46.013 19285-19285/? E/AndroidRuntime: Process: com.dusandimitrijevic.dontforget, PID: 19285<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:392)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.widget.ArrayAdapter.getDropDownView(ArrayAdapter.java:415)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.AppCompatSpinner$DropDownAdapter.getDropDownView(AppCompatSpinner.java:621)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.AppCompatSpinner$DropDownAdapter.getView(AppCompatSpinner.java:617)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.AppCompatSpinner.compatMeasureContentWidth(AppCompatSpinner.java:544)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.AppCompatSpinner.access$500(AppCompatSpinner.java:62)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.AppCompatSpinner$DropdownPopup.computeContentWidth(AppCompatSpinner.java:735)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.AppCompatSpinner$DropdownPopup.show(AppCompatSpinner.java:760)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.AppCompatSpinner$1.onForwardingStarted(AppCompatSpinner.java:260)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.ListPopupWindow$ForwardingListener.onLongPress(ListPopupWindow.java:1430)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.ListPopupWindow$ForwardingListener.access$1000(ListPopupWindow.java:1248)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.support.v7.widget.ListPopupWindow$ForwardingListener$TriggerLongPress.run(ListPopupWindow.java:1521)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.os.Handler.handleCallback(Handler.java:739)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:95)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:145)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5951)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:372)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)<br />
01-21 02:45:46.013 19285-19285/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)</pre>

This is an error i'm getting and i really do not know where i'm making a mistake. Here is my code where i'm setting Spinner and custom adapter to spinner .. I'm getting an error when i click on spinner item. I'm getting spinner item in activity, but when i want to see drop down menu with spinner items, i'm getting the error from above.

<pre>public class EventActivity extends AppCompatActivity implements
        View.OnClickListener, AdapterView.OnItemClickListener, AdapterView.OnItemLongClickListener {

    // VARIABLES
    private AlertDialog.Builder builder;
    private DataBaseHelper db;
    private EventListAdapter adapter;
    private Date mDate;
    private EditText inputName;
    private ListView list;
    private TextView txtDate;
    private SimpleDateFormat formatter = new SimpleDateFormat("MMMM dd", Locale.ENGLISH);
    private DatePickerDialog datePickerDialog;
    private Calendar dateCalendar;
    private List<String> categories;
    private Spinner spinnerCategories;

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

        builder = new AlertDialog.Builder(this);
        LayoutInflater inflater = (LayoutInflater) EventActivity.this.getSystemService(LAYOUT_INFLATER_SERVICE);
        View view = inflater.inflate(R.layout.custom_dialog_header, null);
        builder.setCustomTitle(view);

        initViews();
        setAdapter(mDate);
        setDatePickerDialog();
        setSpinner();
        String selectedItem = spinnerCategories.getSelectedItem().toString();

    }

    private void setSpinner() {
        spinnerCategories = (Spinner) findViewById(R.id.categories);

        // SPINNER DROP DOWN ELEMENTS
        categories = new ArrayList<>();
        categories.add("Birthday");
        categories.add("Anniversary");
        categories.add("18th Birthday");
        categories.add("1th Birthday");

        // CREATING ADAPTER FOR SPINNER
        ArrayAdapter<String> dataAdapter = new ArrayAdapter<>(EventActivity.this, R.layout.simple_spinner_item, R.id.list_item, categories);
        dataAdapter.setDropDownViewResource(R.layout.simpler_spinner_dropdown_item);
        spinnerCategories.setAdapter(dataAdapter);
    }
// other code

HERE ARE SOME XML FILES:

simple_spinner_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/list_item"
        style="@style/spinnerItemStyle"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:ellipsize="marquee"
        android:singleLine="true" />

</LinearLayout>

simple_spinner_dropdown_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <CheckedTextView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@android:id/text1"
        style="@style/spinnerDropDownItemStyle"
        android:layout_width="match_parent"
        android:layout_height="?attr/dropdownListPreferredItemHeight"
        android:ellipsize="marquee"
        android:singleLine="true" />

</LinearLayout>