There are times that If the engine fails to evaluate objects being null, your game or project can potentially crash, or when you are working in the editor, upon stopping the simulation, you receive pop-up errors (Blueprint Runtime Error).
In this article, we focus on “Is Valid”, an important utility in blueprint scripting which sometimes tends to be underestimated and results in various errors and crashes.
https://dev.epicgames.com/community/learning/tutorials/ow3B/unreal-engine-blueprint-is-valid-check