SSMS crashes when attempting to create a database diagram
I've found that the database diagram tool seems to crash if you modify things outside of the diagram.
Diagrams appear to remain stable if:

The database diagram tool is used to do all of your modifications, thus retaining all metadata of the process within the diagram.
Your database structure is modified outside of the diagram which is then deleted and recreated each time.
The entire database is built, ground up, to a point that you are satisfied and feel that it won't change.  The database should then be diagrammed post-beta.

Whichever method is used, this is a known issue.
To confirm, I have this issue while using SSMS 18.4 against a SQL 2016 Express instance.
Current Status - 10/03/2021 - SSMS Version 18.7.1
I still have issues with the diagram tool. It is slightly more stable, though, if you modify anything outside of a current diagram, the editor complains about metadata and you lose your work. In fact, I found that I had to also close and reopen SSMS to ensure I could edit or create a diagrams.  So, while SSMS remains a little more stable (that's debatable), whatever has been done is not a solution.
Roll on version 37.n - let's hope it's fixed by then!
Further Notes From a Small Island (12/03/2021)
There also appears to be an issue with putting your PC to sleep, particularly if you're working over a VPN. I found that when you sleep your computer, then attempt to modify a diagram that was left open, the editor errors with various meta-data related faults (there has been more than one fault). The remedy to this was to reconnect the VPN, then refresh the database diagrams folder in SSMS...

This should allow you to save the diagram.