Unity, weird error appearing after playing in editor

got a bit of a weird error, hopefully someone here can help.

So I have created a custom class (does not derive from Monobehavior).

Then I have a variable for the class, in the Start() method I set it to 'null', first time I play it in editor, everything is fine, (pickups/weapon switching etc..), but then when I play it again after a while, the variable is not null, even though I set it to null in Start().

Here's the code to make it easier:

[System.Serializable]
public class Weapon
{
    public string weaponName;
}

public class PlayerFPS : MonoBehaviour
{
   public Weapon secondary;

   void Start(){
      secondary = null;
      print(secondary == null); //this returns True the first time I play, but False after
   }
}

It seems that for some reason after playing in the editor, the variable stops setting to 'null'.

I do a lot of Instantiating and Destroying, so perhaps, the issue is somehow related to Garbage Collector or Cache?

Thanks in advance!