fix(Pool): Return now checks for null entries to avoid nulls creeping into the pool

Assets/Mirror/Core/Tools/Pool.cs

@@ -32,7 +32,15 @@ public Pool(Func<T> objectGenerator, int initialCapacity)
 
         // return an element to the pool
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void Return(T item) => objects.Push(item);
+        public void Return(T item)
+        {
+            // make sure we can't accidentally insert null values into the pool.
+            // debugging this would be hard since it would only show on get().
+            if (item == null)
+                throw new ArgumentNullException(nameof(item));
+
+            objects.Push(item);
+        }
 
         // count to see how many objects are in the pool. useful for tests.
         public int Count => objects.Count;

Assets/Mirror/Tests/Editor/Tools/PoolTests.cs

@@ -34,6 +34,15 @@ public void ReturnAndTake()
             Assert.That(pool.Get(), Is.EqualTo("returned"));
         }
 
+        [Test]
+        public void ReturnNull()
+        {
+            // make sure we can't accidentally insert null values into the pool.
+            // debugging this would be hard since it would only show on get().
+            Assert.That(() => pool.Return(null), Throws.ArgumentNullException);
+            Assert.That(pool.Count, Is.EqualTo(0));
+        }
+
         [Test]
         public void Count()
         {