Hello Eva,
I created a simple test page, based on the code you have provided, but unfortunately, I was not able to reproduce the problem you describe - the RadGrid was visible as soon as the RadToolTip showed. I have attached my test page for your reference. Could you please prepare and send us a simple running project, demonstrating the problem, so that we can research what is causing it?
Sincerely yours,
Tsvetie
the Telerik team
Check out Telerik Trainer, the state of the art learning tool for Telerik products.