The Problem is Specific to Vivaldi on KDE, the link I provided was solved So, you can try the same Solution in the link, if it does not work purge KDE 
I'm not on KDE, I just have it on my computer. The link was unsolved. The link said unsolved.
I fixed that by purging KDE and fiddling with keyboard settings. Guess it overrode.
So, Everything is Fine Now?
My original problem stays though. Thundar opens. It might not be because of KDE but it's opening now.
And it's not even installed.
What is Thunder?
To Check If Thunar is Installed or Not.
Press Alt + F2
Type thunar,
If it's Present click launch
You Confused me too 
Your PC is Currently running(KDE/Gnome)?
What Application is Giving you the Problem and does the Problem replicates in other applications?
Sorry, sorry!
So I am on Vivaldi, I recently installed KDE and then didn't use it. Today I opened it and then went back to Gnome.
Vivaldi's file upload, when I was sending something I had to friends, opened the Dolphin (I think) file manager instead of the old one.
Okay, So Your Problem is that you use Vivaldi and thunar (default file manager) is not opening files using the thunar (black one) and using Dolphin (white one) which you removed. This is Purely a Vivaldi problem, not of Zorin/Ubuntu/ any other desktop environment Problem.
Follow this guide to report a Bug because there is nothing we can do here and this problem was reported by other Vivaldi users.
This Shall give you a Solution from Our Side 
So I changed the desktop to Zorin on Wayland and then back to Zorin desktop and it looks fixed. Thanks for your time, sorry if I was unclear.
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