fix(material-experimental/mdc-slider): fix a few null pointer exceptions (#23659)

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.

Author: crisbeto

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

@@ -46,12 +46,13 @@ 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;
@@ -98,6 +99,12 @@ 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

@@ -852,32 +852,32 @@ export class MatSlider
 
   /** 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?.some(input => input._disabled) || false;
     this._setDisabled(disabled);
   }
 
   /** Gets the slider thumb input of the given thumb position. */
   _getInput(thumbPosition: Thumb): MatSliderThumb {
-    return thumbPosition === Thumb.END ? this._inputs.last! : this._inputs.first!;
+    return thumbPosition === Thumb.END ? this._inputs?.last! : this._inputs?.first!;
   }
 
   /** Gets the slider thumb HTML input element of the given thumb position. */
   _getInputElement(thumbPosition: Thumb): HTMLInputElement {
-    return this._getInput(thumbPosition)._hostElement;
+    return this._getInput(thumbPosition)?._hostElement;
   }
 
   _getThumb(thumbPosition: Thumb): MatSliderVisualThumb {
-    return thumbPosition === Thumb.END ? this._thumbs.last! : this._thumbs.first!;
+    return thumbPosition === Thumb.END ? this._thumbs?.last! : this._thumbs?.first!;
   }
 
   /** Gets the slider thumb HTML element of the given thumb position. */
   _getThumbElement(thumbPosition: Thumb): HTMLElement {
-    return this._getThumb(thumbPosition)._getHostElement();
+    return this._getThumb(thumbPosition)?._getHostElement();
   }
 
   /** Gets the slider knob HTML element of the given thumb position. */
   _getKnobElement(thumbPosition: Thumb): HTMLElement {
-    return this._getThumb(thumbPosition)._getKnob();
+    return this._getThumb(thumbPosition)?._getKnob();
   }
 
   /**
@@ -1167,7 +1167,7 @@ class SliderAdapter implements MDCSliderAdapter {
     evtType: K,
     handler: SpecificEventListener<K>,
   ): void => {
-    this._delegate._getThumbElement(thumbPosition).removeEventListener(evtType, handler);
+    this._delegate._getThumbElement(thumbPosition)?.removeEventListener(evtType, handler);
   };
   registerInputEventHandler = <K extends EventType>(
     thumbPosition: Thumb,
@@ -1177,7 +1177,7 @@ class SliderAdapter implements MDCSliderAdapter {
     if (evtType === 'change') {
       this._saveChangeEventHandler(thumbPosition, handler as SpecificEventListener<EventType>);
     } else {
-      this._delegate._getInputElement(thumbPosition).addEventListener(evtType, handler);
+      this._delegate._getInputElement(thumbPosition)?.addEventListener(evtType, handler);
     }
   };
   deregisterInputEventHandler = <K extends EventType>(
@@ -1188,7 +1188,7 @@ class SliderAdapter implements MDCSliderAdapter {
     if (evtType === 'change') {
       this._globalEventSubscriptions.unsubscribe();
     } else {
-      this._delegate._getInputElement(thumbPosition).removeEventListener(evtType, handler);
+      this._delegate._getInputElement(thumbPosition)?.removeEventListener(evtType, handler);
     }
   };
   registerBodyEventHandler = <K extends EventType>(