The control implicitly uses a resource specified in its meta: attribute, and a value for this property is defined in the ...

The control implicitly uses a resource specified in its meta: attribute, and a value for this property is defined in the resource (for example, in a .resx file in the App_LocalResource folder). To create an explicit resource expression for this property, either remove the control's meta: attribute in Source view, or remove the property value in the resource.