Locate the definition of the theme specified by the android:theme attribute
within your application’s style resources. This theme specifies the
launch theme of your application. Ensure that the style attribute configures the
android:windowBackground attribute with your custom splash screen. For example:

    
<resources>
    <style
        name="SomeTheme"
        // ...
        >
        <!-- Show a splash screen on the activity. Automatically removed when
             Flutter draws its first frame -->
        <item name="android:windowBackground">@drawable/some_splash_screen</item>
    </style>
</resources>