My code works in Preview Mode, but not in my Application

If you have been moving elements, look for an element that is not on the form, but is on the canvas (the surface under the form). When you move a layout element off the form in Designer, you will see the legend "Parenting to Canvas (Ghosting)" Runtime has no canvas. Designer does. Therefore, when code compiles in Designer it can "see" that element; when it compiles in runtime, it cannot. Moving that element back onto the form should allow the code to compile correctly in both Runtime and Designer. To select all Ghosted elements on the canvas, right-click anywhere on the canvas itself (and outside of a layout element), and choose "Select All Ghosted Elements".