Thank you for your post.
I was unable to reproduce this behavior with FileMaker Pro 14.0.4 (Advanced) on Windows 10 Pro (64-bit / Build 10240). To reproduce I added 20 items onto my toolbar from both the Customizable and Standard items. I did not experience a crash or inconsistency.
Have you had this issue when dragging items on the Toolbar in a new file?
If you have further information needed to reproduce, just let me know.
Crash does NOT occur when I open an empty file.
On further testing it seems the crash only occurs when modify the tool bar in a particular one of my hosted database, which I will shall cal "crash inducer".
I can open another database hosted on the same server, modify the tool bar then close that DB and open "crash inducer" modified tool bar works fine, but any attempt to modify tool bar while open to crash inducer results in a FMPA locking up, customize window shows "Not Responding"
Thank you for your response.
Since the problem only occurs in a single file, I would recommend Recovering the "crash inducer" file. This will need to be done locally with a copy of FileMaker Pro 14. Recover will attempt to report and can sometimes even resolve damage in a database file. If you find that the file is damaged, you might check Knowledge Base Article #5421: What to do when your file is corrupt.
I hope this information is of use.
If your theme is "Cool Gray", try changing it to something else. Worked for me.