Worked like a charm!!
Thank you so very much for your advice!!
I have EXACTLY the same situation as ddierkes had encountered.
After the "ADB push error: " message below:
My nexus 5 is stuck like the screenshot in the original post.
Mobile: Nexus 5 (brand new and just unpacked)
Mobile OS: pre-installed original Android OS
Computer OS: Ubuntu 18.04
UBports Installer: ubports-installer_0.1.12-beta_amd64
I can see adb (137.5kb) and fastboot(93.1kb) files in /usr/bin/.
Now it's impossible to turn off my Nexus 5 (Google logo and lock icon always come back).
I'd be grateful if ddierkes or somebody could let me know how to solve the issue.