I have stumbled upon a possible fix for you.
At least it fixed the issue for me and I can now open the track icon palette again.
There are a handful of folders in the ~User/Music/Audio Music Apps folder.
One in particular, /Channel Strip Settings caught my eye, so I took the steps below.
• Quit Logic Pro
• Renamed the "Channel Strip Settings" folder to "renamed Channel Strip Settings", and left it in place.
So now had the folder ~User/Music/Audio Music Apps/renamed Channel Strip Settings.
• Launched Logic Pro 12 and opened an existing project. Made a minor edit to the project and then Quit Logic without saving the project.
• I now saw that Logic had created a new "Channel Strip Settings" folder, apparently as replacement for the default folder that I had previously renamed. I deleted this new folder replacement.
• Returned to the previously renamed folder and restored its name to Channel Strip Settings.
• Launched Logic Pro again, opened an existing project and can now open the track icon palette from both the track header and the Track inspector panel. The icon palette also continues to work when I create a new project.
I hope this works for you, too.