Not able to create in place RTP plugin which add extra info to extension

Hi, I have a simple rtp filter plugin (rtpcustom) with 'application/x-rtp' caps on sink and source. The whole purpose of this plugin is to add some data to the extension. As sink and source caps are the same, I am using transform_ip (overriding BaseTransform).

Below is transform_ip :

impl BaseTransformImpl for RtpCustom {
    const MODE: gst_base::subclass::BaseTransformMode =
        gst_base::subclass::BaseTransformMode::AlwaysInPlace;
    const PASSTHROUGH_ON_SAME_CAPS: bool = false;
    const TRANSFORM_IP_ON_PASSTHROUGH: bool = false;

    fn transform_ip(&self, element: &Self::Type, buf: &mut gst::BufferRef) -> Result<gst::FlowSuccess, gst::FlowError> {
        let mut new_buf = unsafe {gst::Buffer::from_glib_full(buf.as_mut_ptr())};
        if (new_buf.is_writable()) {
            let mut rtp_buffer =  RTPBuffer::from_buffer_writable(&mut new_buf).unwrap();
            let had_extension = rtp_buffer.is_extension();
            let extension_data: [u8; 4] = [100, 101, 102, 103];
            rtp_buffer.add_extension_onebyte_header(1, &extension_data).map_err(|_| {
                gst::element_error!(element,
                    gst::ResourceError::Read,
                    ["Add extension one byte failed"]
                );
                gst::FlowError::Error
            });
            gst_debug!(
                CAT,
                obj: element,
                "Wrote to copy for buf {} : {} : {:?}", had_extension, rtp_buffer.is_extension(), rtp_buffer
            );
        }
        Ok(gst::FlowSuccess::Ok)
    }
}

When i run pipeline GST_DEBUG="rtpt:7" gst-launch-1.0 rtspsrc location="rtsp://127.0.0.1:8090/stream" ! rtpcustom ! decodebin ! autovideosink I get the following error

(gst-launch-1.0:57976): GStreamer-CRITICAL **: 16:34:36.512: gst_pad_push: assertion 'GST_IS_BUFFER (buffer)' failed
ERROR: from element /GstPipeline:pipeline0/GstRTSPSrc:rtspsrc0/GstUDPSrc:udpsrc0: Internal data stream error.

I also tried let mut new_buf = unsafe {gst::Buffer::from_glib_none(buf.as_mut_ptr())}; But in this case, buffer is not writable.