fix(autocomplete): throw better error when autocomplete doesn't have a panel (#4851)

Adds a more informative error if the user tries to open an autocomplete that doesn't exist or hasn't been defined yet by the `@ViewChild` query.

Fixes #4844.

src/lib/autocomplete/autocomplete-trigger.ts

@@ -51,6 +51,15 @@ export const MD_AUTOCOMPLETE_VALUE_ACCESSOR: any = {
   multi: true
 };
 
+/**
+ * Creates an error to be thrown when attempting to use an autocomplete trigger without a panel.
+ */
+export function getMdAutocompleteMissingPanelError(): Error {
+  return new Error('Attempting to open an undefined instance of `md-autocomplete`. ' +
+                   'Make sure that the id passed to the `mdAutocomplete` is correct and that ' +
+                   'you\'re attempting to open it after the ngAfterContentInit hook.');
+}
+
 @Directive({
   selector: 'input[mdAutocomplete], input[matAutocomplete]',
   host: {
@@ -124,6 +133,10 @@ export class MdAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
 
   /** Opens the autocomplete suggestion panel. */
   openPanel(): void {
+    if (!this.autocomplete) {
+      throw getMdAutocompleteMissingPanelError();
+    }
+
     if (!this._overlayRef) {
       this._createOverlay();
     } else {
@@ -177,7 +190,7 @@ export class MdAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
 
   /** The currently active option, coerced to MdOption type. */
   get activeOption(): MdOption {
-    if (this.autocomplete._keyManager) {
+    if (this.autocomplete && this.autocomplete._keyManager) {
       return this.autocomplete._keyManager.activeItem as MdOption;
     }
   }

src/lib/autocomplete/autocomplete.spec.ts

@@ -10,15 +10,19 @@ import {
 } from '@angular/core';
 import {By} from '@angular/platform-browser';
 import {NoopAnimationsModule} from '@angular/platform-browser/animations';
-import {MdAutocompleteModule, MdAutocompleteTrigger} from './index';
+import {FormControl, FormsModule, ReactiveFormsModule} from '@angular/forms';
+import {
+  MdAutocompleteModule,
+  MdAutocompleteTrigger,
+  MdAutocomplete,
+  getMdAutocompleteMissingPanelError,
+} from './index';
 import {OverlayContainer} from '../core/overlay/overlay-container';
 import {MdInputModule} from '../input/index';
 import {Dir, LayoutDirection} from '../core/rtl/dir';
-import {FormControl, FormsModule, ReactiveFormsModule} from '@angular/forms';
 import {Subscription} from 'rxjs/Subscription';
 import {ENTER, DOWN_ARROW, SPACE, UP_ARROW, ESCAPE} from '../core/keyboard/keycodes';
 import {MdOption} from '../core/option/option';
-import {MdAutocomplete} from './autocomplete';
 import {MdInputContainer} from '../input/input-container';
 import {Observable} from 'rxjs/Observable';
 import {Subject} from 'rxjs/Subject';
@@ -26,9 +30,9 @@ import {dispatchFakeEvent} from '../core/testing/dispatch-events';
 import {createKeyboardEvent} from '../core/testing/event-objects';
 import {typeInElement} from '../core/testing/type-in-element';
 import {ScrollDispatcher} from '../core/overlay/scroll/scroll-dispatcher';
-
 import 'rxjs/add/operator/map';
 
+
 describe('MdAutocomplete', () => {
   let overlayContainerElement: HTMLElement;
   let dir: LayoutDirection;
@@ -50,7 +54,8 @@ describe('MdAutocomplete', () => {
         NgIfAutocomplete,
         AutocompleteWithNgModel,
         AutocompleteWithOnPushDelay,
-        AutocompleteWithNativeInput
+        AutocompleteWithNativeInput,
+        AutocompleteWithoutPanel
       ],
       providers: [
         {provide: OverlayContainer, useFactory: () => {
@@ -1156,6 +1161,15 @@ describe('MdAutocomplete', () => {
       expect(fixture.componentInstance.mdOptions.length).toBe(2);
     }));
 
+    it('should throw if the user attempts to open the panel too early', async(() => {
+      const fixture = TestBed.createComponent(AutocompleteWithoutPanel);
+      fixture.detectChanges();
+
+      expect(() => {
+        fixture.componentInstance.trigger.openPanel();
+      }).toThrow(getMdAutocompleteMissingPanelError());
+    }));
+
   });
 
   it('should have correct width when opened', () => {
@@ -1422,3 +1436,11 @@ class AutocompleteWithNativeInput {
     });
   }
 }
+
+
+@Component({
+  template: `<input placeholder="Choose" [mdAutocomplete]="auto">`
+})
+class AutocompleteWithoutPanel {
+  @ViewChild(MdAutocompleteTrigger) trigger: MdAutocompleteTrigger;
+}