fix: Add ILPP error-handling to catch managed INetworkSerializables that don't meet the new() constraint so they won't crash the editor [MTT-6211] (#2528)

Author: ShadauxCat

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -17,6 +17,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 - Fixed issue where a client could throw an exception if abruptly disconnected from a network session with one or more spawned `NetworkObject`(s). (#2510)
 - Fixed issue where invalid endpoint addresses were not being detected and returning false from NGO UnityTransport. (#2496)
 - Fixed some errors that could occur if a connection is lost and the loss is detected when attempting to write to the socket. (#2495)
+- Making a `NetworkVariable` with an `INetworkSerializable` type that doesn't meet the `new()` constraint will now create a compile-time error instead of an editor crash (#2528)
 - Fixed the inspector throwing exceptions when attempting to render `NetworkVariable`s of enum types. (#2529)
 - Fixed an exception and error logging when two different objects are shown and hidden on the same frame (#2524)
 

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

@@ -210,6 +210,21 @@ private void CreateNetworkVariableTypeInitializers(AssemblyDefinition assembly)
                         {
                             if (type.HasInterface(typeof(INetworkSerializable).FullName))
                             {
+                                var constructors = type.Resolve().GetConstructors();
+                                var hasEmptyConstructor = false;
+                                foreach (var constructor in constructors)
+                                {
+                                    if (constructor.Parameters.Count == 0)
+                                    {
+                                        hasEmptyConstructor = true;
+                                    }
+                                }
+
+                                if (!hasEmptyConstructor)
+                                {
+                                    m_Diagnostics.AddError($"{type} cannot be used in a network variable - Managed {nameof(INetworkSerializable)} instances must meet the `new()` (default empty constructor) constraint.");
+                                    continue;
+                                }
                                 serializeMethod = new GenericInstanceMethod(m_NetworkVariableSerializationTypes_InitializeSerializer_ManagedINetworkSerializable_MethodRef);
                             }