fix(datepicker): make sure _datepickerInput exists before accessing its (#7033)

* fix(datepicker): make sure _datepickerInput exists before accessing its
disabled property.

* add test

src/lib/datepicker/datepicker.spec.ts

@@ -304,6 +304,10 @@ describe('MatDatepicker', () => {
         fixture.detectChanges();
       }));
 
+      it('should not throw when accessing disabled property', () => {
+        expect(() => testComponent.datepicker.disabled).not.toThrow();
+      });
+
       it('should throw when opened with no registered inputs', async(() => {
         expect(() => testComponent.datepicker.open()).toThrow();
       }));

src/lib/datepicker/datepicker.ts

@@ -147,7 +147,8 @@ export class MatDatepicker<D> implements OnDestroy {
   /** Whether the datepicker pop-up should be disabled. */
   @Input()
   get disabled() {
-    return this._disabled === undefined ? this._datepickerInput.disabled : this._disabled;
+    return this._disabled === undefined && this._datepickerInput ?
+        this._datepickerInput.disabled : this._disabled;
   }
   set disabled(value: any) {
     const newValue = coerceBooleanProperty(value);