How to judge whether a class reference is NULL in blueprint?

Development Programming & Scripting Blueprint
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How to judge whether a class reference is NULL in blueprint?

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eXi gives a good explanation here on his post:
https://forums.unrealengine.com/showthread.php?67936-TUTORIALS-Short-Tutorials-for-a-better-understanding-of-coding

check the 2 videos to start with, hope it helps.