use property rather than getter for hero party

Assets/Scripts/HeroUnit.cs

@@ -1,6 +1,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
+using UnityEngine.Assertions;
 
 using System.Runtime.CompilerServices;
 [assembly: InternalsVisibleTo("TestsPlaymode")]
@@ -16,21 +17,17 @@ public class HeroUnit : MonoBehaviour
     [SerializeField] private long level;
     [SerializeField] private long xp;
     [SerializeField] private HeroAttributesSO attrs;
-    [SerializeField] private HeroPartySO party;
+    [SerializeField] private HeroPartySO _party;
+    internal HeroPartySO party => _party;
 
     private void OnEnable()
     {
-        // NOTE(zeph): Party is mandatory. If none is set, we want an exception.
-        party.Add(this);
+        Assert.IsNotNull(_party);
+        _party.Add(this);
     }
 
     private void OnDisable()
     {
-        party.Remove(this);
-    }
-
-    internal HeroPartySO getParty()
-    {
-        return party;
+        _party.Remove(this);
     }
 }