Fixed errors when a parameter is destroyed and it's reference is still used in the graph.

Author: alelievr

Assets/com.alelievr.NodeGraphProcessor/Runtime/Elements/ExposedParameter.cs

@@ -68,7 +68,7 @@ void ISerializationCallbackReceiver.OnBeforeSerialize() {}
         protected virtual Settings CreateSettings() => new Settings();
 
         public virtual object value { get; set; }
-        public virtual Type GetValueType() => value.GetType();
+        public virtual Type GetValueType() => value == null ? typeof(object) : value.GetType();
 
         static Dictionary<Type, Type> exposedParameterTypeCache = new Dictionary<Type, Type>();
         internal ExposedParameter Migrate()

Assets/com.alelievr.NodeGraphProcessor/Runtime/Elements/ParameterNode.cs

@@ -97,6 +97,13 @@ protected override void Process()
 			parameter = graph.GetExposedParameterFromGUID(parameterGUID);
 #endif
 
+			ClearMessages();
+			if (parameter == null)
+			{
+				AddMessage($"Parameter not found: {parameterGUID}", NodeMessageType.Error);
+				return;
+			}
+
 			if (accessor == ParameterAccessor.Get)
 				output = parameter.value;
 			else

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed fields with [Settings] attribute not showing up with inheritance.
 - Fixed selection still active when selecting nodes without inspector fields.
 - Fixed multi-selection drag and drop of edges.
+- Fixed errors when a parameter is destroyed and it's reference is still used in the graph.
 
 ### Changed
 - Fields with both attributes [SerializeField] and [ShowInInspector] are now visible in both the node and inspector.