fix(material-experimental/mdc-slider): fix a few null pointer exceptions

Fixes a few null pointer errors if:
1. `setDisabledState` is called too early. This can't happen at the moment, but it showed up when trying to fix something in `@angular/forms` and it may become an issue in the future.
2. The component is destroyed before the initialization is complete. This showed up while investigating the first error.

src/material-experimental/mdc-slider/slider.spec.ts

@@ -55,11 +55,12 @@ describe('MDC-based MatSlider' , () => {
   }
 
   describe('standard slider', () => {
+    let fixture: ComponentFixture<StandardSlider>;
     let sliderInstance: MatSlider;
     let inputInstance: MatSliderThumb;
 
     beforeEach(waitForAsync(() => {
-      const fixture = createComponent(StandardSlider);
+      fixture = createComponent(StandardSlider);
       fixture.detectChanges();
       const sliderDebugElement = fixture.debugElement.query(By.directive(MatSlider));
       sliderInstance = sliderDebugElement.componentInstance;
@@ -105,6 +106,13 @@ describe('MDC-based MatSlider' , () => {
       expect(inputInstance.value).toBe(10);
       sliderInstance._elementRef.nativeElement.style.marginLeft = 'initial';
     });
+
+    it('should not throw if destroyed before initialization is complete', () => {
+      fixture.destroy();
+      fixture = TestBed.createComponent(StandardSlider);
+      expect(() => fixture.destroy()).not.toThrow();
+    });
+
   });
 
   describe('standard range slider', () => {

src/material-experimental/mdc-slider/slider.ts

@@ -814,7 +814,7 @@ export class MatSlider extends _MatSliderMixinBase
 
   /** Sets the disabled state based on the disabled state of the inputs (ControlValueAccessor). */
   _updateDisabled(): void {
-    const disabled = this._inputs.some(input => input._disabled);
+    const disabled = this._inputs ? this._inputs.some(input => input._disabled) : false;
     this._setDisabled(disabled);
   }
 
@@ -1112,21 +1112,23 @@ class SliderAdapter implements MDCSliderAdapter {
   }
   deregisterThumbEventHandler = <K extends EventType>
     (thumbPosition: Thumb, evtType: K, handler: SpecificEventListener<K>): void => {
-      this._delegate._getThumbElement(thumbPosition).removeEventListener(evtType, handler);
+      if (this._delegate._initialized) {
+        this._delegate._getThumbElement(thumbPosition).removeEventListener(evtType, handler);
+      }
   }
   registerInputEventHandler = <K extends EventType>
     (thumbPosition: Thumb, evtType: K, handler: SpecificEventListener<K>): void => {
       if (evtType === 'change') {
         this._saveChangeEventHandler(thumbPosition, handler as SpecificEventListener<EventType>);
-      } else {
+      } else if (this._delegate._initialized) {
         this._delegate._getInputElement(thumbPosition).addEventListener(evtType, handler);
       }
   }
   deregisterInputEventHandler = <K extends EventType>
     (thumbPosition: Thumb, evtType: K, handler: SpecificEventListener<K>): void => {
       if (evtType === 'change') {
         this._globalEventSubscriptions.unsubscribe();
-      } else {
+      } else if (this._delegate._initialized) {
         this._delegate._getInputElement(thumbPosition).removeEventListener(evtType, handler);
       }
   }