Lightroom Classic CC stopped opening files in Photoshop CC. Selecting "Edit in..Adobe Photoshop CC 2109" opens photoshop but no photo opens. This has crippled my entire workflow. LR & PShop are both latest versions. I unintsalled & reinstalled both, losing all my prefs, and stilll have the same problem. iMac Pro, os 10.14 Mojave, latest versions.
I had the same issue and what I have done is opened both photoshop and Lightroom initially. now when I want to edit from Lightroom to photoshop it works as usual. If photoshop is not already open when trying to go from lightroom to photoshop you won't be able to edit anything in photoshop. A bit of a work around but it works...
I have since discovered it's a problem ONLY with three specific DNG files. Others are opening normally. While I can open those three directly in Photoshop, they continue refusing to open in Photoshop from Lightroom. I have run Disk Utility and Disk Warrior on that SSD and there appear to be no errors. Another 'puter mystery.
Mac users: have you tried running Onyx to clean out caches and Application States? Run the free Onyx (https://www.titanium-software.fr/en/onyx.html), reboot. (In Onyx just run everything in the Automation tab). Run Disk Warrior if you can ($120), (https://www.alsoft.com), reboot. If you can’t do Disk Warrior, at least run First Aid from Disk Utility. Also just doing a "Safe Boot" can clear out caches (restart, hold down shift until you see "Safe Boot" on the screen, then reboot normally.