Improve generating both columns and dataitems in grid needs a dummy item

[zdravkov]

Currently in order to have both column and items generated we need to add a dummy column item initially as in this item - https://stackblitz.com/edit/j1w3ix?file=src/main.vue

We need to think of improvement of some warning in these cases to provide a better experience.

[PekoPPT]

Requested in ticket 1605107.