Accessed none trying to read property

Hello Unreal Forum,

I’m new here, my name is Tom, i’m german so my english is not so good, sorry for that.
In the little scene the ThirdPersonCharacter runs into the box and the light should go on.
But there is an error:
Blueprint Runtime Error: “Accessed None trying to read property Var_Door_Button”. Blueprint: Door_Panel Function: Execute Ubergraph Door Panel Graph: EventGraph Node: Door Open Close"
In the Screenshots see the details.
Does everyone know how to solve this?
Here is the link of this unreal project:

best regards

Hello Stormrage256,
thank you.
Here is the Screenshot with the definition of “Var_Door_Button”.
How to avoid the null and how to get the variable the desired Value?