fix(ivy): ngTemplateOutlet error when switching between null and template value (#32160)

Fixes an error that is thrown by `ngTemplateOutlet` under Ivy when switching from a template to null and back to a template. The error is thrown because the reference to the previous ViewRef is never cleared and the directive tries to detach a view that has already been detached.

Fixes #32060.

PR Close #32160

packages/common/src/directives/ng_template_outlet.ts

@@ -55,20 +55,19 @@ export class NgTemplateOutlet implements OnChanges {
     const recreateView = this._shouldRecreateView(changes);
 
     if (recreateView) {
+      const viewContainerRef = this._viewContainerRef;
+
       if (this._viewRef) {
-        this._viewContainerRef.remove(this._viewContainerRef.indexOf(this._viewRef));
+        viewContainerRef.remove(viewContainerRef.indexOf(this._viewRef));
       }
 
-      if (this.ngTemplateOutlet) {
-        this._viewRef = this._viewContainerRef.createEmbeddedView(
-            this.ngTemplateOutlet, this.ngTemplateOutletContext);
-      }
-    } else {
-      if (this._viewRef && this.ngTemplateOutletContext) {
+      this._viewRef = this.ngTemplateOutlet ?
+          viewContainerRef.createEmbeddedView(this.ngTemplateOutlet, this.ngTemplateOutletContext) :
+          null;
+    } else if (this._viewRef && this.ngTemplateOutletContext) {
       this._updateExistingContext(this.ngTemplateOutletContext);
     }
   }
-  }
 
   /**
    * We need to re-create existing embedded view if:
@@ -96,9 +95,8 @@ export class NgTemplateOutlet implements OnChanges {
         }
       }
       return false;
-    } else {
-      return true;
     }
+    return true;
   }
 
   private _updateExistingContext(ctx: Object): void {

packages/common/test/directives/ng_template_outlet_spec.ts

@@ -218,6 +218,25 @@ describe('NgTemplateOutlet', () => {
     }).not.toThrow();
   });
 
+  it('should not throw when switching from template to null and back to template', async(() => {
+       const template = `<tpl-refs #refs="tplRefs"><ng-template>foo</ng-template></tpl-refs>` +
+           `<ng-container [ngTemplateOutlet]="currentTplRef"></ng-container>`;
+       fixture = createTestComponent(template);
+       fixture.detectChanges();
+       const refs = fixture.debugElement.children[0].references !['refs'];
+
+       setTplRef(refs.tplRefs.first);
+       detectChangesAndExpectText('foo');
+
+       setTplRef(null);
+       detectChangesAndExpectText('');
+
+       expect(() => {
+         setTplRef(refs.tplRefs.first);
+         detectChangesAndExpectText('foo');
+       }).not.toThrow();
+     }));
+
 });
 
 @Injectable()