About mean mesh modeling

[Bereket Workie Anteneh]

I am doing SSM on proximal femur bone, I have 25 datasets in STL file format and created correspondence based on Parametric,  non-rigid registration (in tutorial 12). Mean while,  I want to see the mean deformation and mean mesh from the given datasets based on the code provided on tutorial 3 (https://scalismo.org/docs/tutorials/tutorial3). Unfortunately, the following error shown in GUI of the Scalismo. So, how I can fix the error shown?