android: Prevent setup fragment crash in background

Sometimes during onSaveInstanceState, the SetupFragment would crash the app in the background if we tried to store the state of a view.

src/android/app/src/main/java/org/yuzu/yuzu_emu/fragments/SetupFragment.kt

@@ -295,8 +295,10 @@ class SetupFragment : Fragment() {
 
     override fun onSaveInstanceState(outState: Bundle) {
         super.onSaveInstanceState(outState)
+        if (_binding != null) {
             outState.putBoolean(KEY_NEXT_VISIBILITY, binding.buttonNext.isVisible)
             outState.putBoolean(KEY_BACK_VISIBILITY, binding.buttonBack.isVisible)
+        }
         outState.putBooleanArray(KEY_HAS_BEEN_WARNED, hasBeenWarned)
     }