Background: Add safety check for parent block

Mainly to test the background in isolation in a new Control scene.

Author: manuq

addons/block_code/ui/blocks/utilities/background/background.gd

@@ -56,7 +56,7 @@ func _ready():
 
 
 func _get_border_color() -> Color:
-	if parent_block.has_focus():
+	if parent_block and parent_block.has_focus():
 		return Constants.FOCUS_BORDER_COLOR
 	return outline_color