The screen manager does not clear any graphic set to null

[NicoleYarroch]

Bug Report

If the user tries to clear an image from the screen by setting an image field to null using the screen manager, the image is never cleared. This is due to the blank artwork always being null.

In the TextAndGraphicManager class:

SdlArtwork getBlankArtwork() {
    if (blankArtwork != null){
        ...
    }
    return blankArtwork;
}

should be:

SdlArtwork getBlankArtwork() {
    if (blankArtwork == null){ /* <- The blank artwork object should be created if not already */
        ...
    }
    return blankArtwork;
}

Reproduction Steps

1. Create an app using the screen manager. Set the primary graphic field to an image.
2. Connect the app to a head unit. The image from step 1 should show up in the primary graphic field
3. Set the primary graphic field to null using the screen manager.

Expected Behavior

The primary graphic image field should be cleared.

Observed Behavior

The image from step 1 stays visible.

OS & Version Information

• Android Version: 9, API 28
• SDL Android Version: 4.7
• Testing Against: SDL_Core 5.0 and generic_hmi

[BrettyWhite]

Closed with #954