I have a shower knob which controls both hot/cold water. Recently it has been loose. This morning it came off in my hand when I turned it to turn on the shower. Nothing appears to be broken. Basically there is the knob which has a screw coming out of it and some white, plastic thing that appears to slide over where the screw would go. Is there any special way I must put this back on? It seems obvious to take the cap off of the knob, hold the white thing in place and screw it on. Is this ok? Thanks for any and all help!