[IMP] app: throw error when static components key is missing in parent

This commit improves the error message that's thrown when a static
component definition is missing, as outlined in issue #1286.

src/runtime/app.ts

@@ -151,7 +151,13 @@ export class App<
       } else {
         // new component
         if (isStatic) {
-          C = parent.constructor.components[name as any];
+          const components = parent.constructor.components;
+          if (!components) {
+            throw new OwlError(
+              `Cannot find the definition of component "${name}", missing static components key in parent`
+            );
+          }
+          C = components[name as any];
           if (!C) {
             throw new OwlError(`Cannot find the definition of component "${name}"`);
           } else if (!(C.prototype instanceof Component)) {

tests/components/__snapshots__/error_handling.test.ts.snap

@@ -38,6 +38,21 @@ exports[`basics display a nice error if it cannot find component 1`] = `
 }"
 `;
 
+exports[`basics display a nice error if the components key is missing with subcomponents 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  const comp1 = app.createComponent(\`MissingChild\`, true, false, false, true);
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    const b2 = comp1({}, key + \`__1\`, node, this, null);
+    return block1([], [b2]);
+  }
+}"
+`;
+
 exports[`basics no component catching error lead to full app destruction 1`] = `
 "function anonymous(app, bdom, helpers
 ) {

tests/components/error_handling.test.ts

@@ -126,6 +126,23 @@ describe("basics", () => {
     );
   });
 
+  test("display a nice error if the components key is missing with subcomponents", async () => {
+    class Parent extends Component {
+      static template = xml`<div><MissingChild /></div>`;
+    }
+    const app = new App(Parent as typeof Component);
+    let error: Error;
+    const mountProm = app.mount(fixture).catch((e: Error) => (error = e));
+    await expect(nextAppError(app)).resolves.toThrow(
+      'Cannot find the definition of component "MissingChild", missing static components key in parent'
+    );
+    await mountProm;
+    expect(error!).toBeDefined();
+    expect(error!.message).toBe(
+      'Cannot find the definition of component "MissingChild", missing static components key in parent'
+    );
+  });
+
   test("simple catchError", async () => {
     class Boom extends Component {
       static template = xml`<div t-esc="a.b.c"/>`;