Using the Npgsql data provider of version 6.0.7 leads to the following error when there is a DateTime SQL parameter:

Can't write CLR type System.String with handler type TimestampTzHandler or Can't write CLR type System.String with handler type TimestampHandler.

This error is thrown only on Design-Time because the Design-Time value for the SQL parameter does not respect its selected DbType and is instead always a string.