I have this issue sometimes when i start the editor through the shortcut on my desktop,
what i do then is starting it through the exe in the folders of the game itself. And then
the editor runs fine. This .exe is in my case here;
C:\Program Files (x86)\Ubisoft\Far Cry 5\bin
exe is called;
ArcadeEditor64.exe
i assume its the same in your case, maybe you have the game installed on a different drive,
then you need to look there. I am quit sure this will fix it for you, at least i hope it does.
Much success,
Leon