Unity doesn't recognize my slider after loading a new scene (Scene not done loading?) What to do?

Hi, i got this function that gets called when pressing the setting button in my menu. now it should put the slider value to the speed value that was saved before (with the same slider) but i get a NullReferenceException. If I activate it manually a bit later with a button it works just fine. What can I do? Thanks for your kind help!