fix: add better error messages for a couple of error cases

- When using a generic INetworkSerializable+IEquatable combo and update documentation with workaround
- When there are no initialized elements in the network prefab list (fix null reference exception)

Author: larus

com.unity.netcode.gameobjects/Documentation~/basics/networkvariable.md

@@ -621,3 +621,55 @@ public class TestFixedString : NetworkBehaviour
 
 > [!NOTE]
 > The above example uses a pre-set list of strings to cycle through for example purposes only.  If you have a predefined set of text strings as part of your actual design then you would not want to use a FixedString to handle synchronizing the changes to `m_TextString`.  Instead, you would want to use a `uint` for the type `T` where the `uint` was the index of the string message to apply to `m_TextString`.
+
+## Generic IEquatable network variables
+
+Generic `INetworkSerializable` types with generic `IEquatable` are not supported, implemented as `public class NotSupported<T> : INetworkSerializable, IEquatable<NotSupported<T>>` where the type would be passed in during declaration like `NetworkVariable<NotSupported<int>> myVar;`.
+
+The recommended workaround for this would be to create the generic class as usual but add a virtual method for handling the serialization of the type. Then wrap this generic `INetworkSerializable` in a derived class which then needs to have a serializable type defined where the implementation for the serialization is provided. 
+
+For example:
+
+```csharp
+public class FirstGenClass<T> : INetworkSerializable
+{
+    // This needs to be a serializable type according to what network variables support
+    public T Data;
+
+    protected virtual void OnNetworkSerialize<T2>(BufferSerializer<T2> serializer) where T2 : IReaderWriter
+    {
+    }
+
+    public void NetworkSerialize<T2>(BufferSerializer<T2> serializer) where T2 : IReaderWriter
+    {
+        OnNetworkSerialize(serializer);
+    }
+}
+
+public class SecondGenWithLong : FirstGenClass<long>, IEquatable<SecondGenWithLong>
+{
+    // Potential additional data
+    public int AdditionalData;
+
+    protected virtual bool OnEquals(SecondGenWithLong other)
+    {
+        return other.Data.Equals(other);
+    }
+    public bool Equals(SecondGenWithLong other)
+    {
+        return OnEquals(other);
+    }
+
+    protected override void OnNetworkSerialize<T2>(BufferSerializer<T2> serializer)
+    {
+        serializer.SerializeValue(ref AdditionalData);
+        serializer.SerializeValue(ref Data);
+    }
+}
+```
+
+Then declare this network variable like so:
+
+```csharp
+NetworkVariable<SecondGenWithLong> myVar = new NetworkVariable<SecondGenWithLong>();
+```

com.unity.netcode.gameobjects/Editor/CodeGen/NetworkBehaviourILPP.cs

@@ -409,7 +409,14 @@ private void CreateNetworkVariableTypeInitializers(AssemblyDefinition assembly,
                     }
                     else
                     {
-                        m_Diagnostics.AddError($"{type}: Managed type in NetworkVariable must implement IEquatable<{type}>");
+                        foreach (var typeInterface in type.Resolve().Interfaces)
+                        {
+                            if (typeInterface.InterfaceType.Name.Contains(typeof(IEquatable<>).Name) && typeInterface.InterfaceType.IsGenericInstance)
+                            {
+                                m_Diagnostics.AddError($"{type}: A generic IEquatable '{typeInterface.InterfaceType.FullName}' is not supported.");
+                            }
+                        }
+                        m_Diagnostics.AddError($"{type}: Managed type in NetworkVariable must implement IEquatable<{type}>.");
                         equalityMethod = new GenericInstanceMethod(m_NetworkVariableSerializationTypes_InitializeEqualityChecker_ManagedClassEquals_MethodRef);
                     }
 

com.unity.netcode.gameobjects/Editor/NetworkManagerEditor.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using Unity.Netcode.Editor.Configuration;
 using UnityEditor;
 using UnityEngine;
@@ -301,6 +302,10 @@ private void DisplayNetworkManagerProperties()
                     {
                         EditorGUILayout.HelpBox("You have no prefab list selected. You will have to add your prefabs manually at runtime for netcode to work.", MessageType.Warning);
                     }
+                    else if (m_NetworkManager.NetworkConfig.Prefabs.NetworkPrefabsLists.All(x => x == null))
+                    {
+                        EditorGUILayout.HelpBox("All prefab lists selected are uninitialized. You will have to add your prefabs manually at runtime for netcode to work.", MessageType.Warning);
+                    }
                     EditorGUILayout.PropertyField(m_PrefabsList);
                 }
 

com.unity.netcode.gameobjects/Runtime/Configuration/NetworkPrefabs.cs

@@ -81,6 +81,10 @@ internal void Shutdown()
         {
             foreach (var list in NetworkPrefabsLists)
             {
+                if (list == null)
+                {
+                    continue;
+                }
                 list.OnAdd -= AddTriggeredByNetworkPrefabList;
                 list.OnRemove -= RemoveTriggeredByNetworkPrefabList;
             }
@@ -96,6 +100,10 @@ public void Initialize(bool warnInvalid = true)
             m_Prefabs.Clear();
             foreach (var list in NetworkPrefabsLists)
             {
+                if (list == null)
+                {
+                    continue;
+                }
                 list.OnAdd += AddTriggeredByNetworkPrefabList;
                 list.OnRemove += RemoveTriggeredByNetworkPrefabList;
             }
@@ -109,6 +117,10 @@ public void Initialize(bool warnInvalid = true)
             {
                 foreach (var list in NetworkPrefabsLists)
                 {
+                    if (list == null)
+                    {
+                        continue;
+                    }
                     prefabs.AddRange(list.PrefabList);
                 }
             }

com.unity.netcode.gameobjects/Tests/Editor/NetworkManagerConfigurationTests.cs

@@ -294,6 +294,36 @@ public void WhenModifyingPrefabListUsingPrefabsAPI_ModificationIsLocal()
             }
         }
 
+        [Test]
+        public void WhenThereAreUninitializedElementsInPrefabsList_NoErrors()
+        {
+            // Setup
+            var networkManagerObject = new GameObject();
+            var networkManager = networkManagerObject.AddComponent<NetworkManager>();
+            networkManager.NetworkConfig = new NetworkConfig
+            {
+                NetworkTransport = networkManager.gameObject.AddComponent<UnityTransport>()
+            };
+
+            try
+            {
+                networkManager.NetworkConfig.Prefabs.NetworkPrefabsLists = new List<NetworkPrefabsList> { null };
+                networkManager.Initialize(true);
+
+                Assert.IsTrue(networkManager.NetworkConfig.Prefabs.NetworkPrefabsLists.Count == 1);
+                Assert.IsTrue(networkManager.NetworkConfig.Prefabs.NetworkPrefabsLists[0] == null);
+                Assert.IsTrue(networkManager.NetworkConfig.Prefabs.Prefabs.Count == 0);
+            }
+            finally
+            {
+                networkManager.ShutdownInternal();
+                // Shutdown doesn't get called correctly because we called Initialize()
+                // instead of calling StartHost/StartClient/StartServer. See MTT-860 for
+                // why.
+                networkManager.NetworkConfig?.NetworkTransport.Shutdown();
+            }
+        }
+
         [Test]
         public void WhenModifyingPrefabListUsingPrefabsListAPI_ModificationIsShared()
         {