gstreamer and ubuntu, NULL returned from getting xcontext

[Davide Perini]

Hi guys,
I installed gstreamer in Ubuntu 20.04 as mentioned in the official site:

|apt-get install libgstreamer1.0-0 gstreamer1.0-plugins-base 
gstreamer1.0-plugins-good gstreamer1.0-plugins-bad 
gstreamer1.0-plugins-ugly gstreamer1.0-libav gstreamer1.0-doc 
gstreamer1.0-tools gstreamer1.0-x gstreamer1.0-alsa gstreamer1.0-gl 
gstreamer1.0-gtk3 gstreamer1.0-qt5 gstreamer1.0-pulseaudio|


when I try to screen capture:
gst-launch-1.0 ximagesrc ! videoscale ! autovideosink

I get an error.

---
root at sblantipodi-Virtual-Machine:~# gst-launch-1.0 ximagesrc ! 
videoscale ! autovideosink
Setting pipeline to PAUSED ...
error: XDG_RUNTIME_DIR not set in the environment.
ERROR: Pipeline doesn't want to pause.
ERROR: from element /GstPipeline:pipeline0/GstXImageSrc:ximagesrc0: 
Could not open X display for reading
Additional debug info:
gstximagesrc.c(158): gst_ximage_src_open_display (): 
/GstPipeline:pipeline0/GstXImageSrc:ximagesrc0:
NULL returned from getting xcontext
Setting pipeline to NULL ...
Freeing pipeline ...
---

Can you help me please?
Same command works well on Fedora.

Thank you!
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