I'm having the same trouble...
The older versions of the Nextion Editor are under Announcements in the thread
DO NOT Install an older version when you have a new version of the Nextion Editor
Instead the disk1.cab file needs to be extracted from the installation file and then
unpacked into a clean folder.
1) Create a new folder on your system
2) Copy the installation folder to your new folder
3) from the command line, change directory to your new folder
4) extract disk1.cab with > nextion-setup-v0.38.exe /extract
5) with 7zip or equivalent, unpack the disk1.cab file
6) remove the setup exe file (to avoid accidental installation)
7) Run the older version Nextion Editor from that new folder.
8) Without opening any projects, click Debug
- this will ask you to select a TFT file to run.
If you are uncomfortable with any of these steps, DO NOT PROCEED.
The best option is to ask the author if they are willing to share their HMI file.