Proxy: white clip and video part missing

STEPS TO REPRODUCE

1. Enable and create proxy clip
2. disable video stream (clip properties) -> result no.1
3. disable proxy -> result no.2

OBSERVED RESULT

No.1: a white clip appears
No.2: only audio is available. No chance to bring back the video.

ADDITIONAL INFORMATION

White clip: We should make a warning and guide the user how to enable the video in the properties.