If I open IntelliJ IDEA and try and import any project or open any folder containing a project, nothing happens, I'm just returned to this screen.
Earlier on, before I moved my project files around, I tried to set a new JDK for a project and i clicked add SDK and then selected Java 12 home and again it returned to the screen without adding anything.
Is there some permission I need to grant to IntelliJ Idea?
Seems to have started since installing MacOS Catalina.
It's a known issue specific to the recent macOS Catalina update. Please follow YouTrack ticket for updates.
You can also help by providing the following information in YouTrack:
Open the file chooser, select a directory (but don't open it yet), take a screenshot, then open a directory; then attach idea.log and the screenshot (a path to a project and a time of the operation are needed, too, but they should be guessable from the screenshot).
As a workaround until this issue is resolved you can add -Dide.mac.file.chooser.native=false
in Help | Edit Custom VM Options and restart the IDE.