fix: Prevent Gutenberg connection logic from crashing Aztec

A network connectivity subscriber was added to the post activity in: #19692

However, it was not scoped to only run in the Gutenberg editor. Because
no guard was in place, the subscriber attempted to invoke a non-existent
method on the Aztec editor, resulting in a crash.

WordPress/src/main/java/org/wordpress/android/ui/posts/EditPostActivity.java

@@ -71,6 +71,7 @@
 import org.wordpress.android.editor.EditorMediaUtils;
 import org.wordpress.android.editor.EditorThemeUpdateListener;
 import org.wordpress.android.editor.ExceptionLogger;
+import org.wordpress.android.editor.gutenberg.GutenbergNetworkConnectionListener;
 import org.wordpress.android.editor.savedinstance.SavedInstanceDatabase;
 import org.wordpress.android.editor.gutenberg.DialogVisibility;
 import org.wordpress.android.editor.gutenberg.GutenbergEditorFragment;
@@ -3844,6 +3845,8 @@ public void onEventMainThread(UploadService.UploadMediaRetryEvent event) {
 
     @Subscribe(threadMode = ThreadMode.MAIN)
     public void onEventMainThread(ConnectionChangeReceiver.ConnectionChangeEvent event) {
+        if (!(mEditorFragment instanceof GutenbergNetworkConnectionListener)) return;
+
         ((GutenbergEditorFragment) mEditorFragment).onConnectionStatusChange(event.isConnected());
     }