Fix a crash with media

app/src/main/java/app/fedilab/android/mastodon/ui/fragment/media/FragmentMedia.java

@@ -110,9 +110,13 @@ public class FragmentMedia extends Fragment {
                 enableSliding(true);
             }
         });
-        binding.mediaPicture.setOnClickListener(v -> ((MediaActivity) requireActivity()).toogleFullScreen());
+        binding.mediaPicture.setOnClickListener(v -> {
+            if(isAdded()){((MediaActivity) requireActivity()).toogleFullScreen();}
+        });
 
-        binding.mediaVideo.setOnClickListener(v -> ((MediaActivity) requireActivity()).toogleFullScreen());
+        binding.mediaVideo.setOnClickListener(v -> {
+            if(isAdded()) {((MediaActivity) requireActivity()).toogleFullScreen();}
+        });
 
         String type = attachment.type;
         String preview_url = attachment.preview_url;