A null event handler delegate cannot be subscribed to an event type.

Resolved https://github.com/space-wizards/RobustToolbox/issues/575.

Robust.Shared/GameObjects/EntityManager.cs

@@ -348,6 +348,10 @@ namespace Robust.Shared.GameObjects
         public void SubscribeEvent<T>(EntityEventHandler<T> eventHandler, IEntityEventSubscriber s)
             where T : EntityEventArgs
         {
+            // adding a null handler delegate should do nothing, since trying to invoke it would throw a NullRefException
+            if(eventHandler == null)
+                return;
+
             var eventType = typeof(T);
             if (!_eventSubscriptions.TryGetValue(eventType, out var subscriptions))
             {

Robust.UnitTesting/Shared/GameObjects/EntityManager_Tests.cs

@@ -0,0 +1,52 @@
+using System;
+using NUnit.Framework;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Interfaces.GameObjects;
+using Robust.Shared.Map;
+
+namespace Robust.UnitTesting.Shared.GameObjects
+{
+    [TestFixture, Parallelizable, TestOf(typeof(EntityManager))]
+    public class EntityManager_Tests
+    {
+        /// <summary>
+        /// Raising a null C# delegate does not generate a NullReferenceException.
+        /// </summary>
+        [Test]
+        public void SubscribeEvent_NullEvent_NoNullException()
+        {
+            // Arrange
+            var manager = new TestEntityManager();
+            var subscriber = new TestEventSubscriber();
+
+            manager.SubscribeEvent((EntityEventHandler<TestEventArgs>) null, subscriber);
+            
+            // Act
+            manager.RaiseEvent(null, new TestEventArgs());
+
+            //Assert: this should do nothing
+        }
+    }
+
+    internal class TestEventSubscriber : IEntityEventSubscriber { }
+
+    internal class TestEntityManager : EntityManager
+    {
+        public override IEntity SpawnEntity(string protoName)
+        {
+            throw new NotImplementedException();
+        }
+
+        public override IEntity SpawnEntityNoMapInit(string protoName)
+        {
+            throw new NotImplementedException();
+        }
+
+        public override IEntity SpawnEntityAt(string entityType, GridCoordinates coordinates)
+        {
+            throw new NotImplementedException();
+        }
+    }
+
+    internal class TestEventArgs : EntityEventArgs { }
+}