register PreviewPost for RPC connections (#18452)

plugin/client_rpc.go

@@ -168,6 +168,7 @@ func init() {
 	gob.Register(&model.AutocompleteDynamicListArg{})
 	gob.Register(&model.AutocompleteStaticListArg{})
 	gob.Register(&model.AutocompleteTextArg{})
+	gob.Register(&model.PreviewPost{})
 }
 
 // These enforce compile time checks to make sure types implement the interface