pgAdmin3 can't connect properly to Postgres 9.2
Other than not using features that cause errors, no, there is no workaround. Since in this case one of the features that causes an error appears to be listing tablespaces during startup, you're out of luck.

Just compile a newer version of PgAdmin-III from sources or use the command-line psql client that comes with PostgreSQL 9.2.

PostgreSQL's system catalogs aren't 100% compatible from version to version. That's intentional; if they had to be exactly the same it'd be very hard to make changes and improvements to PostgreSQL. Programs that use the system catalogs should be prepared to require updates when a new PostgreSQL version comes out. Most tools that use the catalogs - like psql, PgAdmin-III, pg_dump, etc - do so because it's the only way to get detailed information about some of the inner workings of the system. You just need to update them when you update the server.