fix(cdk/drag-drop): error if dragged item is destroyed as a result of the entered event (#22904)

When an item enters a new drop container, we dispatch an event and run some logic afterwards. The problem is that if the item is destroyed as a result of the event, the logic after the event will throw because dragging was interrupted.

Fixes #22813.

(cherry picked from commit 0431d81)

Author: crisbeto

src/cdk/drag-drop/directives/drag.spec.ts

@@ -5447,6 +5447,28 @@ describe('CdkDrag', () => {
       expect(itemEnterEvent).toEqual(expectedEvent);
     }));
 
+    it('should not throw if dragging was interrupted as a result of the entered event',
+      fakeAsync(() => {
+        const fixture = createComponent(ConnectedDropZones);
+        fixture.detectChanges();
+
+        const groups = fixture.componentInstance.groupedDragItems;
+        const item = groups[0][1];
+        const targetRect = groups[1][2].element.nativeElement.getBoundingClientRect();
+
+        fixture.componentInstance.enteredSpy.and.callFake(() => {
+          fixture.componentInstance.todo = [];
+          fixture.detectChanges();
+        });
+
+        expect(() => {
+          dragElementViaMouse(
+            fixture, item.element.nativeElement, targetRect.left + 1, targetRect.top + 1);
+          flush();
+          fixture.detectChanges();
+        }).not.toThrow();
+      }));
+
     it('should be able to drop into a new container after scrolling into view', fakeAsync(() => {
       const fixture = createComponent(ConnectedDropZones);
       fixture.detectChanges();

src/cdk/drag-drop/drag-ref.ts

@@ -151,7 +151,7 @@ export class DragRef<T = any> {
    * Whether the dragging sequence has been started. Doesn't
    * necessarily mean that the element has been moved.
    */
-  private _hasStartedDragging: boolean;
+  private _hasStartedDragging = false;
 
   /** Whether the element has moved since the user started dragging it. */
   private _hasMoved: boolean;
@@ -982,10 +982,13 @@ export class DragRef<T = any> {
       });
     }
 
-    this._dropContainer!._startScrollingIfNecessary(rawX, rawY);
-    this._dropContainer!._sortItem(this, x, y, this._pointerDirectionDelta);
-    this._applyPreviewTransform(
-      x - this._pickupPositionInElement.x, y - this._pickupPositionInElement.y);
+    // Dragging may have been interrupted as a result of the events above.
+    if (this.isDragging()) {
+      this._dropContainer!._startScrollingIfNecessary(rawX, rawY);
+      this._dropContainer!._sortItem(this, x, y, this._pointerDirectionDelta);
+      this._applyPreviewTransform(
+        x - this._pickupPositionInElement.x, y - this._pickupPositionInElement.y);
+    }
   }
 
   /**