Fix inspector field serialization

- Implement handling for OdinSerialize on UdonSharpProgramAsset since it's no longer working
- Add error for attempting to compile a program asset without a source file specified

Author: MerlinVR

Assets/UdonSharp/Editor/UdonSharpCompilationModule.cs

@@ -37,6 +37,9 @@ public CompilationModule(UdonSharpProgramAsset sourceAsset)
 
         public int Compile(List<ClassDefinition> classDefinitions)
         {
+            if (programAsset.sourceCsScript == null)
+                throw new System.ArgumentException($"Asset '{AssetDatabase.GetAssetPath(programAsset)}' does not have a valid program source to compile from");
+
             sourceCode = File.ReadAllText(AssetDatabase.GetAssetPath(programAsset.sourceCsScript));
 
             SyntaxTree tree = CSharpSyntaxTree.ParseText(sourceCode);

Assets/UdonSharp/Editor/UdonSharpProgramAsset.cs

@@ -41,6 +41,9 @@ void Start()
         [NonSerialized, OdinSerialize]
         public Dictionary<string, FieldDefinition> fieldDefinitions;
 
+        [SerializeField, HideInInspector]
+        private SerializationData serializationData;
+
         private static bool showProgramUasm = false;
 
         protected override void DrawProgramSourceGUI(UdonBehaviour udonBehaviour, ref bool dirty)
@@ -514,6 +517,18 @@ protected override object DrawPublicVariableField(string symbol, object variable
 
             return variableValue;
         }
+
+        protected override void OnBeforeSerialize()
+        {
+            UnitySerializationUtility.SerializeUnityObject(this, ref serializationData);
+            base.OnBeforeSerialize();
+        }
+
+        protected override void OnAfterDeserialize()
+        {
+            UnitySerializationUtility.DeserializeUnityObject(this, ref serializationData);
+            base.OnAfterDeserialize();
+        }
     }
 
     [CustomEditor(typeof(UdonSharpProgramAsset))]