Cast boolean from widget blueprint to pawn blueprint

Within my PlayMenu blueprint the user can select one of 6 buttons which determines the material used to skin the ball pawn in the RollingBall blueprint, but I keep getting the error:

Error Blueprint Runtime Error: Accessed None trying to read property CallFunc_Array_Get_Item from function: ‘ExecuteUbergraph_PlayMenu’ from node: Set bool1 in graph: EventGraph in object: PlayMenu with description: Accessed None trying to read property CallFunc_Array_Get_Item

I have tried using ‘Cast To’, but it says it is not required as RollingBall is already referenced.
I have also tried calling different custom events but I can’t seem to get any of them to work.

PlayMenu blueprint:

RollingBall blueprint:

thanks for your help.