fix: exiting whiteboard crash

calling `requireContext()` after leaving the fragment caused a crash.

Fixed by storing the value of whether the system is using gestures navigation instead of retrieving it every time.

If the user changes the gestures navigation mode while using the reviewer, the activity and fragment are recreated, so that ends up not being a problem

java.lang.IllegalStateException: Fragment ReviewerFragment{a6a61db} (47dfdd3d-a8b4-49be-ad89-2e879c2630b8) not attached to a context.
at androidx.fragment.app.Fragment.requireContext(Fragment.java:977)
at com.ichi2.anki.ui.windows.reviewer.ReviewerFragment.setupWhiteboard$lambda$1(ReviewerFragment.kt:554)
at com.ichi2.anki.ui.windows.reviewer.ReviewerFragment.setupWhiteboard$lambda$1(ReviewerFragment.kt:554)

Author: BrayanDSO

AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerFragment.kt

@@ -545,13 +545,13 @@ class ReviewerFragment :
             },
             false,
         )
+        val isUsingGesturesNavigation = compat.isUsingSystemGestureNavigation(requireContext())
         val doubleBackCallback =
             doubleBackPressCallback(
                 enabled = false,
                 onFirstBack = { showSnackbar(R.string.back_pressed_once, Snackbar.LENGTH_SHORT) },
                 shouldReEnable = {
-                    viewModel.whiteboardEnabledFlow.value &&
-                        compat.isUsingSystemGestureNavigation(requireContext())
+                    viewModel.whiteboardEnabledFlow.value && isUsingGesturesNavigation
                 },
             )
         requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner, doubleBackCallback)