check for no actors in sequence diagram

d2layouts/d2sequence/layout.go

@@ -106,8 +106,11 @@ func layoutSequenceDiagram(g *d2graph.Graph, obj *d2graph.Object) (*sequenceDiag
 		}
 	}
 
-	sd := newSequenceDiagram(obj.ChildrenArray, edges)
+	sd, err := newSequenceDiagram(obj.ChildrenArray, edges)
-	err := sd.layout()
+	if err != nil {
+		return nil, err
+	}
+	err = sd.layout()
 	return sd, err
 }
 

d2layouts/d2sequence/sequence_diagram.go

@@ -1,6 +1,7 @@
 package d2sequence
 
 import (
+	"errors"
 	"fmt"
 	"math"
 	"sort"
@@ -64,7 +65,7 @@ func getEdgeEarliestLineNum(e *d2graph.Edge) int {
 	return min
 }
 
-func newSequenceDiagram(objects []*d2graph.Object, messages []*d2graph.Edge) *sequenceDiagram {
+func newSequenceDiagram(objects []*d2graph.Object, messages []*d2graph.Edge) (*sequenceDiagram, error) {
 	var actors []*d2graph.Object
 	var groups []*d2graph.Object
 
@@ -84,6 +85,10 @@ func newSequenceDiagram(objects []*d2graph.Object, messages []*d2graph.Edge) *se
 		}
 	}
 
+	if len(actors) == 0 {
+		return nil, errors.New("no actors declared in sequence diagram")
+	}
+
 	sd := &sequenceDiagram{
 		messages:        messages,
 		actors:          actors,
@@ -185,7 +190,7 @@ func newSequenceDiagram(objects []*d2graph.Object, messages []*d2graph.Edge) *se
 		sd.maxActorHeight += float64(*sd.root.LabelHeight)
 	}
 
-	return sd
+	return sd, nil
 }
 
 func (sd *sequenceDiagram) layout() error {

e2etests/regression_test.go

@@ -37,6 +37,7 @@ group.nested: {
   a -> b
 }
 `,
+			expErr: "no actors declared in sequence diagram",
 		},
 		{
 			name: "nested_steps",