Android – how do I best place my fuzzy ProgressBar on the constraint layout on the screen?

When I click the "login" button, I will see a progress bar. I modified the layout by adding a toolbar at the top. Now it seems that the toolbar blurs the progress bar

How do I display this progress bar somewhere in the center of the screen?

Thank you very much for your help

The layout XML code is as follows:

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:layout_marginTop="16dp"
android:gravity="center"
android:orientation="vertical">

<android.support.v7.widget.Toolbar
    android:id="@+id/my_toolbar"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:layout_marginBottom="64dp"
    android:background="@android:color/white"
    android:elevation="4dp"
    android:theme="@style/ThemeOverlay.AppCompat.ActionBar"
    app:layout_constraintBottom_toTopOf="@+id/edittext_username"
    app:popupTheme="@style/ThemeOverlay.AppCompat.Light" />

<EditText
    android:id="@+id/edittext_username"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginBottom="24dp"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:drawablePadding="8dp"
    android:gravity="center_vertical"
    android:hint="@string/edittext_username"
    app:layout_constraintBottom_toTopOf="@+id/edittext_password"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="@+id/button_login" />

<EditText
    android:id="@+id/edittext_password"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginBottom="24dp"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:drawablePadding="8dp"
    android:gravity="center_vertical"
    android:hint="@string/edittext_password"
    android:inputType="textPassword"
    app:layout_constraintBottom_toTopOf="@+id/button_login"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.0"
    app:layout_constraintStart_toStartOf="parent" />

<Button
    android:id="@+id/button_login"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginBottom="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:text="@string/button_login"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.0"
    app:layout_constraintStart_toStartOf="parent" />

<ProgressBar
    android:id="@+id/progressbar_login_progress"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="8dp"
    android:visibility="gone" />

</android.support.constraint.ConstraintLayout>

resolvent:

Use the next code to center the progressbar

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >

    <ProgressBar
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/progressBar"
        android:gravity="center"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>