FindClosestEnemy gives Error CS0165: Use of unassigned local variable 'closest'

Hello dear community,

First, sorry for any language mistakes, I originally speak French. Note that I code using C#.

I am working on a game much in the vein of Plants VS Zombies. Everything is going VERY nice so far, but I have a problem making the canonball projectiles (catapult-like motion) to always aim at the closest enemy on the same row (I coded snap-to-grid for drag-and-drop of characters/items). I can easily make everything working, but with an overuse of IF statements, which is definetely bad because I will have to write tons of conditional code for every stage and every enemies wave.

So the problem comes when I try to get rid of overusing all those IF statements; I borrowed a function directly explained in the Unity documentation (ref: here) that checks for the closest enemy. I need to know the closest enemy because my bezier/catapult code needs Start, Mid and End (target) points to create the arc motion for the GameObject, and because I always want the target point of the arc to be the closest enemy on the same row.

However, I get an error "Error CS0165: Use of unassigned local variable 'closest'". I think this can have something to do with the fact that the variable can be null, however I can't figure out what, how and where to define it...

I tried to paste my code with the code button, but seems like I cannot get it to work properly so please check it on pastebin over here.

edit(copied, reformated and extracted relevant parts of the script)

     private GameObject checkNearestItem;
     
     void Update ()
     {
         // [...]
         checkNearestItem = GameObject.Find(FindClosestEnemy().name);
         //Attack the nearest ENEMY     
         if (checkNearestItem != null)
         {
             // [...]
         }
     }
 
     GameObject FindClosestEnemy ()
     {
         GameObject[] gos;
         gos = GameObject.FindGameObjectsWithTag("Croc");
         GameObject closest;
         float distance = Mathf.Infinity;
         Vector3 position = transform.position;
         foreach (GameObject go in gos)
         {
             //List only enemies whose Y position is equal to 90.
             if (go.transform.position.y == 90)
             {
                 Vector3 diff = go.transform.position - position;
                 float curDistance = diff.sqrMagnitude;
                 if (curDistance < distance)
                 { 
                     closest = go;
                     distance = curDistance;
                 }
             }
         }
         return closest;
     }
 

What am I mistaking or doing wrong??

Of course, any help or hint will be highly appreciated... ;)
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Thanks!