Why no message posted when element change state from NULL to READY?

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Hi Developers,

I just tried to use playbin2 play one avi file. And the file can be
played well but from the application side, I can not receive element
ffdec/avidemux state change message from NULL to READY. And I can
receive element state change message from READY to PAUSED, and from
PAUSED to PLAYING.

This is strange for other elements (like streamsynchronizer0,
playsink0, uridecodebin0, typefind, decodebin20, multiqueue0,
inputselector0, ...), I can receive stage change message from NULL to
READY.

Anybody who help me to explain why? And I want to know how to modify
playbin2 then I can get the demux and decoder state change from NULL
to READY.

best regards