[fenix] For https://github.com/mozilla-mobile/fenix/issues/14289: Check if toolbar is initialised before setting flags.

app/src/main/java/org/mozilla/fenix/browser/BaseBrowserFragment.kt

@@ -1162,6 +1162,8 @@ abstract class BaseBrowserFragment : Fragment(), UserInteractionHandler, Session
     }
 
     override fun onAccessibilityStateChanged(enabled: Boolean) {
-        browserToolbarView.setScrollFlags(enabled)
+        if (_browserToolbarView != null) {
+            browserToolbarView.setScrollFlags(enabled)
+        }
     }
 }