I’m not sure it’ll fix your specific issue but what helped me was building the plugin from source with this patch applied: Gst for merging 4 webcams sometimes works, and sometimes it doesn't - #5 by DaneLLL
The above helped me for the compositor side because I saw it had issues with async sources coming up at later times
Besides that, as per my original post, add a check if the attribute pointer exists before accessing it, it’ll go through that loop again later on anyway and set things up correctly