fix(stepper): error being thrown if selected step is accessed too early (#11186)

Fixes an error that is thrown if the selected step is accessed before `AfterViewInit`.

Fixes #11158.

src/cdk/stepper/stepper.ts

@@ -189,9 +189,12 @@ export class CdkStepper implements AfterViewInit, OnDestroy {
 
   /** The step that is selected. */
   @Input()
-  get selected(): CdkStep { return this._steps.toArray()[this.selectedIndex]; }
+  get selected(): CdkStep {
+    // @deletion-target 7.0.0 Change return type to `CdkStep | undefined`.
+    return this._steps ? this._steps.toArray()[this.selectedIndex] : undefined!;
+  }
   set selected(step: CdkStep) {
-    this.selectedIndex = this._steps.toArray().indexOf(step);
+    this.selectedIndex = this._steps ? this._steps.toArray().indexOf(step) : -1;
   }
 
   /** Event emitted when the selected step has changed. */

src/lib/stepper/stepper.spec.ts

@@ -340,6 +340,28 @@ describe('MatStepper', () => {
         selectionChangeSubscription.unsubscribe();
         animationDoneSubscription.unsubscribe();
       }));
+
+    it('should not throw when attempting to get the selected step too early', () => {
+      fixture.destroy();
+      fixture = TestBed.createComponent(SimpleMatVerticalStepperApp);
+
+      const stepperComponent: MatVerticalStepper = fixture.debugElement
+          .query(By.css('mat-vertical-stepper')).componentInstance;
+
+      expect(() => stepperComponent.selected).not.toThrow();
+    });
+
+    it('should not throw when attempting to set the selected step too early', () => {
+      fixture.destroy();
+      fixture = TestBed.createComponent(SimpleMatVerticalStepperApp);
+
+      const stepperComponent: MatVerticalStepper = fixture.debugElement
+          .query(By.css('mat-vertical-stepper')).componentInstance;
+
+      expect(() => stepperComponent.selected = null!).not.toThrow();
+      expect(stepperComponent.selectedIndex).toBe(-1);
+    });
+
   });
 
   describe('icon overrides', () => {