state: move nil check down to stackitem JSON processing

We want to return real errors, not some generic thing for any kind of thing
happening.

pkg/core/state/notification_event.go

@@ -194,10 +194,6 @@ func (e Execution) MarshalJSON() ([]byte, error) {
 	var errRecursive = []byte(`"error: recursive reference"`)
 	arr := make([]json.RawMessage, len(e.Stack))
 	for i := range arr {
-		if e.Stack[i] == nil {
-			arr[i] = errRecursive
-			continue
-		}
 		data, err := stackitem.ToJSONWithTypes(e.Stack[i])
 		if err != nil {
 			data = errRecursive

pkg/core/state/notification_event_test.go

@@ -184,7 +184,13 @@ func TestMarshalUnmarshalJSONAppExecResult(t *testing.T) {
 
 		bs1, err := json.Marshal(actual)
 		require.NoError(t, err)
-		require.Equal(t, bs, bs1)
+		require.NotEqual(t, bs, bs1) // recursive ref error vs. unserializable nil
+
+		actual2 := new(AppExecResult)
+		require.NoError(t, json.Unmarshal(bs, actual2))
+		bs2, err := json.Marshal(actual2)
+		require.NoError(t, err)
+		require.Equal(t, bs1, bs2) // unserializable nil in both cases
 	})
 
 	t.Run("UnmarshalJSON error", func(t *testing.T) {

pkg/vm/stackitem/json.go

@@ -232,10 +232,6 @@ func toJSONWithTypes(item Item, seen map[Item]bool) (interface{}, error) {
 	if len(seen) > MaxJSONDepth {
 		return "", ErrTooDeep
 	}
-	typ := item.Type()
-	result := map[string]interface{}{
-		"type": typ.String(),
-	}
 	var value interface{}
 	switch it := item.(type) {
 	case *Array, *Struct:
@@ -281,6 +277,11 @@ func toJSONWithTypes(item Item, seen map[Item]bool) (interface{}, error) {
 		delete(seen, item)
 	case *Pointer:
 		value = it.pos
+	case nil:
+		return "", fmt.Errorf("%w: nil", ErrUnserializable)
+	}
+	result := map[string]interface{}{
+		"type": item.Type().String(),
 	}
 	if value != nil {
 		result["value"] = value