The error is being caught while Designer is loading extension property definitions imported from the new server connection. These definitions are stored under your workspace .metadata/plugins/org.teiid.designer.extension/imported folder as *.mxd files. If this is an old workspace you may want to clear out this "imported" folder and restart and connect which should import a fresh set of valid extensions for your server.
We're seeing the same issue noted -- cleared out the folder as suggested and issue persists. Any additional comments/thoughts available?