Fix a crash when previewing a node with no output

editor/src/messages/portfolio/document/node_graph/node_graph_message_handler.rs

@@ -3410,7 +3410,8 @@ impl NodeGraphMessageHandler {
 				.exports
 				.iter()
 				.any(|export| matches!(export, NodeInput::Node { node_id: export_node_id, .. } if *export_node_id == node_id));
-			if selection_is_not_already_the_output && no_other_selections {
+			let has_outputs = network_interface.number_of_outputs(&node_id, breadcrumb_network_path) > 0;
+			if selection_is_not_already_the_output && no_other_selections && has_outputs {
 				let button = TextButton::new("Preview")
 					.icon("FrameAll")
 					.tooltip_label("Preview")

editor/src/messages/portfolio/document/utility_types/network_interface/characterization_tests.rs

@@ -3534,3 +3534,18 @@ async fn a_session_preview_of_a_removed_node_is_dropped() {
 		"The evaluated network should fall back to the document's own export"
 	);
 }
+
+#[tokio::test]
+async fn cannot_preview_node_without_outputs() {
+	let mut editor = EditorTestUtils::create();
+	editor.new_document().await;
+
+	let custom_node = editor.create_node_by_name_at(DefinitionIdentifier::Network("Custom Node".to_string()), 0, 0).await;
+	let network_interface = &mut editor.active_document_mut().network_interface;
+
+	assert_eq!(network_interface.number_of_outputs(&custom_node, &[]), 0);
+
+	// Attempting to toggle preview on a node without outputs should not enable previewing
+	network_interface.toggle_preview(custom_node, &[]);
+	assert_eq!(network_interface.previewing(&[]), Previewing::No);
+}

editor/src/messages/portfolio/document/utility_types/network_interface/mutations.rs

@@ -1401,8 +1401,10 @@ impl NodeNetworkInterface {
 		let primary_output = RootNode { node_id: toggle_id, output_index: 0 };
 		let previewing = if self.previewing(network_path).previews(toggle_id) || self.root_node(network_path) == Some(primary_output) {
 			Previewing::No
-		} else {
+		} else if self.number_of_outputs(&toggle_id, network_path) > 0 {
 			Previewing::Yes { previewed: primary_output }
+		} else {
+			return;
 		};
 		self.set_previewing(previewing, network_path);
 	}