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.

(cherry picked from commit 1a03257)

Author: crisbeto

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

@@ -52,12 +52,13 @@ 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;
@@ -104,6 +105,12 @@ 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>(