How can reference to class by class?

I want to make the GameStart of the Puzzle BlockGrid work when the button on the widget class is clicked on. However, when you click on the button, the error ‘blueprint runtime error accessed none try to read property’ appears.
In Unreal docs, the target blueprint was specified in the target menu in Class(self), but there was no target blueprint anywhere.
However, double-click on the Gamestart node of the Widget Class to go to the Gamestart Event of the Puzzle BlockGrid.

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