Panic when trying to use RefreshOptions.Then with mode ASYNC

This doesn't work, and since it took me a while of debugging to figure this out,
alert other developers earlier when they try to do it.

pkg/gui/controllers/helpers/refresh_helper.go

@@ -51,6 +51,10 @@ func NewRefreshHelper(
 }
 
 func (self *RefreshHelper) Refresh(options types.RefreshOptions) error {
+	if options.Mode == types.ASYNC && options.Then != nil {
+		panic("RefreshOptions.Then doesn't work with mode ASYNC")
+	}
+
 	t := time.Now()
 	defer func() {
 		self.c.Log.Infof(fmt.Sprintf("Refresh took %s", time.Since(t)))