Need help with content control of contentPlaceHolder

i made master page and put placeHolder control called SubMenuContent. in content page i had placed a content control called content1. until here everything fine. but if i tried to put anything(like gridview control,label,…etc). in the design view nothing appeared and it gave me handle error in gridview sometimes when it appears

the pic may explain more:
when i tried to drug the gridview control inside content control it didn’t appear in design view. this happens with all tools and controls

i need every control show up in the content control. How?

thanks in advance
Mona