throw error if missing callback in event bus

web/static/src/ts/core/event_bus.ts

@@ -35,6 +35,9 @@ export class EventBus {
    * listener.
    */
   on(eventType: string, owner: any, callback: Callback) {
+    if (!callback) {
+      throw new Error("Missing callback");
+    }
     if (!this.subscriptions[eventType]) {
       this.subscriptions[eventType] = [];
     }

web/static/tests/core/event_bus.test.ts

@@ -20,6 +20,14 @@ describe("event bus behaviour", () => {
     bus.trigger("event");
   });
 
+  test("throw error if callback is undefined", () => {
+    expect.assertions(1);
+    const bus = new EventBus();
+    expect(() => bus.on("event", {}, <any>undefined)).toThrow(
+      `Missing callback`
+    );
+  });
+
   test("can unsubscribe", () => {
     const bus = new EventBus();
     let notified = false;