If you are using Entity Framework and using ODP.NET in your project (from Nuget), please use the same major version of Oracle Developer Tools for Visual Studio (ODT) and ODP.NET.
For example, if you have ODT 19.3, then please use ODP.NET 19.x. If you have ODT 18.3, use ODP.NET 18.x.
In the most recent ODT 19.3 we have a bug that also causes the EF designer to disappear.
We have one workaround for this issue now - if the TNSNAMES.ORA and wallet files are located in the default location this error will not occur.
On installation, the default location is %USERPROFILE%\Oracle\network\admin. The default is also changeable in the options page.
So, you can do one of two things:
1) Backup contents of %USERPROFILE%\Oracle\network\admin, then delete the contents of this directory.
2) Copy the tnsnames.ora, sqlnet.ora and wallet files to this location
3) Restart Visual Studio
1) Go to VS menu, Tools->Options->Oracle Developer Tools ->Connection Configuration and set Tns admin location to the location you want to connect from. Also set wallet file location as well.
2) Restart Visual Studio
This will avoid the crash and allow the EF model to be added to your project, but please be aware that ODP.NET will not automatically be able to find the location of your tnsnames.ora and wallet files. You will need to set TNS_ADMIN and (if needed) WALLET_LOCATION in the config file (eg app.config, or web.config) or include the TNSNAMES/SQLNET.ORA/WALLET/etc in the root of your project. In future releases these values will be set in the ODP.NET connection string automatically and you will not have to do this.