fix(animations): avoid animation insertions during router back/refresh (#21977)

Closes #19712 PR Close #21977

Author: matsko

packages/animations/browser/src/render/transition_animation_engine.ts

@@ -666,7 +666,16 @@ export class TransitionAnimationEngine {
  // code does not contain any animation code in it, but it is
  // just being called so that the node is marked as being inserted
  if (namespaceId) {
- this._fetchNamespace(namespaceId).insertNode(element, parent);
+ const ns = this._fetchNamespace(namespaceId);
+ // This if-statement is a workaround for router issue #21947.
+ // The router sometimes hits a race condition where while a route
+ // is being instantiated a new navigation arrives, triggering leave
+ // animation of DOM that has not been fully initialized, until this
+ // is resolved, we need to handle the scenario when DOM is not in a
+ // consistent state during the animation.
+ if (ns) {
+ ns.insertNode(element, parent);
+ }
  }
 
  // only *directives and host elements are inserted before