EntityFromContext EF Core - entities without primary key error

[lucascaser]

Hello,

I have a .NET core project that has database views mapped to the entity framework and these have the builder.HasNoKey(); configuration, as they are used only for queries.
Besides, I'm using the EntityFromContext() method, but it wasn't handled in case the entity doesn't have a primary key.

It is possible to see that there was no treatment, if the .FindPrimaryKey() == null, generating the error of object reference not set to an instance of an object.

Therefore, I opened a PR dealing with this problem.