I've just installed KDE Neon on Ubuntu 16.04 using PPA. But KNotes becoming black so I can't see the text. I tried to change the background and text color but nothing.
any solution for this..?

I have the same problem. Debian after upgrade from ver. 8 to 9.

One idea would be to copy the text out into a text file, delete and recreate the widget (not using the undo feature as you want a fresh instance of the widget) and then copy the text back into the widget (also as a separate copy from the text file so that it resets any styling that may have been included during the first copy).

It may be tedious if you have a lot of them, but it'll get the job done.

For the intrepid, you can try your hand at editing the config files in ~/.local/share/knotes/ or ~/.local/share/plasma_notes/ (make sure you are logged out of plasma when you do so to ensure that plasma doesn't overwrite your work when you log out)