From 4638833d02edf5776b257c318209616b25527df0 Mon Sep 17 00:00:00 2001
From: Kristiyan Kostadinov <crisbeto@users.noreply.github.com>
Date: Tue, 8 May 2018 18:53:26 +0200
Subject: [PATCH] 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      |  7 +++++--
 src/lib/stepper/stepper.spec.ts | 22 ++++++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/src/cdk/stepper/stepper.ts b/src/cdk/stepper/stepper.ts
index b09d4a87e5c..0c4ada77f92 100644
--- a/src/cdk/stepper/stepper.ts
+++ b/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. */
diff --git a/src/lib/stepper/stepper.spec.ts b/src/lib/stepper/stepper.spec.ts
index 8b63658ec16..72da8f0e36b 100644
--- a/src/lib/stepper/stepper.spec.ts
+++ b/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', () => {