Fix tapping magnification crash in vision settings setup screen

Root cause: ag/17286343 introduces hiding the settings entry in magnification settings if window magnification feature is not supported. It causes SuW page cannot find the settings preference reference.
Solution: Add null pointer check.

Bug: 242242266
Test: Manual testing
Change-Id: I1007a580047dd302228b28c15b82e958a2b01fec
(cherry picked from commit 5a87f6bb)