ExperimentalApi Issue

Hi everyone,
I’m currently working on DroidPlanner Tower and I’m facing something I don’t understand. I want to change programmatically the output of a servo motor. So I saw that there is an “ExperimentalApi” class that allow us to change this value. To test it I replaced the arm button to change servo pwm.
You can see the result:

Caused by: java.lang.ClassNotFoundException: Didn’t find class “org.droidplanner.android.fragments.widget.TowerWidgets”

Can someone help me ?
Thank you guys
Valentin Rudloff