Getting "Accessed None" error and I'm not sure what the cause is

Hi all,
I’m trying to create a small sliding puzzle game from the puzzle template and I already succeeded into making the game works using blueprints, although they are not very clean and still need some work.
The problem is every time I run my game I’m getting arround 140 of these “Accessed None” error which I think doesn’t even point me into the right direction and I’m not sure what the source of my problem is.
If I click on one of the actors on the screen before the errors appear on the log and actor disappear, If I wait then the game works correctly.
I’m adding screenshots of the errors and my main game logic. I hope someone can help me find the cause for these errors or at least show me a good way to find it.

Thanks in advance,

Amit