In VS 2019, ODT install with an extension, not as a setup file like previous version. So, from VS 2017 upgraded EF 6 used MVC project edmx file doesn't show database diagram after extension installation, blank black page, "edit xml" option etc. that's all.
And an error in Error List if you realize!
The ADO.NET provider with invariant name 'Oracle.ManagedDataAccess.Client' is either not registered in the machine or application config file, or could not be loaded. See the inner exception for details
I checked all config files in solution app.config, web.config, etc. everything looks good, all tags and values about "Oracle.ManagedDataAccess.Client" already defined and correct!