findViewById()从一个片段(Android)中返回null。

I'm trying to update progress bars in my app from MainActivity, however it does not let me reference anything that's inside the fragments. If I reference the fragment by using It works but the fragment renders underneath everything (As my app switches fragments out inside a single activity). Anything I've tried returns a NullPointerException on runtime due to the progress bars.I'm trying to update progress bars in my app fr