Maybe @Jitrixis can tell in his own language what he meant to say in line 846, so we can translate it into correct English.

@Mbch331 : I know, I'm not very good at spelling, even in French XD. I will explain my thought.

This warning is fired when you try to edit a query but doesn't exist or has been deleted. Exemple of a JSON response that can fire this warning :

{
    "entities": {
        "Q293856298356": {
            "id": "Q293856298356",
            "missing": ""
        }
    },
    "success": 1
}

In french it would be :

EN : The query is missing from the database. Please create it first
FR : L'entité n'existe pas dans la base de données. Veuillez d'abord la créée.

It is maybe too much technical, we should rephrase it like :

EN : The entity you are about to editing doesn't exist or has been deleted. You need to create it before.
FR : L'entité que vous êtes entrain de modifier n'existe pas ou a été supprimée. Vous devez d'abord la créée.

@Jitrixis Thanks for the explanation. Now I know what happens and rephrase that sentence.

In T107567#1518837, @Jitrixis wrote:

EN : The entity you are about to editing doesn't exist or has been deleted. You need to create it before.
FR : L'entité que vous êtes entrain de modifier n'existe pas ou a été supprimée. Vous devez d'abord la créée.

This message does not make much sense. New entity IDs are automatically assigned by Wikibase, so the best users can try is getting the missing entity restored.

@Ricordisamoa : Yes, maybe, I didn't know that but, it's a warning which appear on first loading data via the API. So its just to handle any possibilities of answer. Secondly, the warning shouldn't appear, because of those lines at the begining :

var itemId = mw.config.get( 'wbEntityId' );
	if ( !itemId ) {
		return;
	}