Why does my Android Dialog throw a NullPointer?

Question

i've been struggling with a simple Android Dialog that I need in order to add elements to a list; it's pretty simple (an edit text, a spinner and a button), but when I try to show it it just keeps throwing a NullPointerException. May I have your help with this matter?

The error:

E/AndroidRuntime: FATAL EXCEPTION: main Process: rmr.kairos, PID: 4519 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:454) at android.widget.ArrayAdapter.getView(ArrayAdapter.java:416) at android.widget.AbsSpinner.onMeasure(AbsSpinner.java:206) at android.widget.Spinner.onMeasure(Spinner.java:614) at androidx.appcompat.widget.AppCompatSpinner.onMeasure(AppCompatSpinner.java:438) at android.view.View.measure(View.java:25466) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957) at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552) at android.widget.LinearLayout.measureVertical(LinearLayout.java:842) at android.widget.LinearLayout.onMeasure(LinearLayout.java:721) at android.view.View.measure(View.java:25466) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at android.view.View.measure(View.java:25466) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at android.view.View.measure(View.java:25466) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6957) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at com.android.internal.policy.DecorView.onMeasure(DecorView.java:747) at android.view.View.measure(View.java:25466) at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:3397) at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:2201) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2486) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1952) at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:8171) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:972) at android.view.Choreographer.doCallbacks(Choreographer.java:796) at android.view.Choreographer.doFrame(Choreographer.java:731) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:957) at android.os.Handler.handleCallback(Handler.java:938) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:223) at android.app.ActivityThread.main(ActivityThread.java:7656) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

The Activity ( @TylerV, I've tried to set an adapter and it does the same, what am I doing wrong?):

public class TagActivity extends AppCompatActivity {
    private ListView lvTag;
    private ImageView imBack;
    private ImageView imNewTag;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_tag);
        lvTag = findViewById(R.id.lvTags);
        imBack = findViewById(R.id.imBack);
        imNewTag = findViewById(R.id.imNewTag);
        imBack.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intentToMain = new Intent(getApplicationContext(), rmr.kairos.activities.MainActivity.class);
                startActivity(intentToMain);
                finish();
            }
        });
        imNewTag.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                showTagDialog();
            }
        });
        newTagList(lvTag);
    }
    public TagAdapter newTagList(ListView lvTag){
        ArrayList<Tag> listaEtiquetas = new ArrayList<Tag>();
        //introducir los valores con un diaálogo o algo
        listaEtiquetas.add(new Tag("prueba", "#FFFFFF"));
        TagAdapter tagAdapter = new TagAdapter(this, R.layout.layout_tag, listaEtiquetas);
        lvTag.setAdapter(tagAdapter);
        return tagAdapter;
    }
public void showTagDialog(){
        AlertDialog.Builder tagBuilder = new AlertDialog.Builder(TagActivity.this);
        View tagView = getLayoutInflater().inflate(R.layout.dialog_tag,null);
        EditText etTagName = tagView.findViewById(R.id.etTagName);
        Spinner spTagColor = tagView.findViewById(R.id.spTagColor);
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(TagActivity.this,
                android.R.layout.simple_spinner_item,
                getResources().getStringArray(R.array.tagSpinnerArray));
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spTagColor.setAdapter(adapter);
        tagBuilder.setPositiveButton("Aceptar", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {
                if (!spTagColor.getSelectedItem().toString().equalsIgnoreCase("Seleccione un color")){
                    Toast.makeText(TagActivity.this, spTagColor.getSelectedItem().toString()
                            , Toast.LENGTH_SHORT).show();
                    dialogInterface.dismiss();
                }
            }
        });
        tagBuilder.setCancelable(true);
        tagBuilder.setView(tagView);
        AlertDialog dialog = tagBuilder.create();
        dialog.show();

    }
} 

The Dialog 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="wrap_content"
    android:layout_margin="10dp"
    android:orientation="vertical"
    android:weightSum="10">

    <TextView
        android:id="@+id/tvDialogTag"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="10dp"
        android:layout_weight="5"
        android:fontFamily="@font/simplifica"
        android:text="@string/strDialogTag"
        android:textAlignment="center"
        android:textSize="24sp"
        android:textStyle="bold" />



        <EditText
            android:id="@+id/etTagName"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:fontFamily="@font/simplifica"
            android:hint="@string/strTagTitle"
            android:shadowColor="#FFFFFF"
            android:textColor="#FFFFFF"
            android:textSize="24sp"
            android:textStyle="bold" />

    <Spinner
        android:id="@+id/spTagColor"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="10dp"
        android:layout_weight="5"
        android:entries="@array/tagSpinnerArray" />

    <Button
        android:id="@+id/btDialogTag"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="right"
        android:layout_weight="5"
        android:text="@string/submit" />
</LinearLayout>

Answer 1

Sadly I couldn't find a solution and my time was running out, so I simply used a radioGroup; not the fanciest of the solutions, but a solution. Thanks everyone!!