application.getUserFromDatabase(getApplicationContext(), username, password);

public void getUserFromDatabase(Context context, String username, String password) {
   //new GetFromDatabase().execute("getUser.php", "?username=" + username + "&password=" + password, this, context);
    WelcomeActivity activity = new WelcomeActivity();
    activity.startMainActivity();
}

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="org.mydomain.app">

    <application
        android:name=".MainApplication"
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">

        </activity>
        <activity android:name=".WelcomeActivity" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>