How do you wait for an item in a blue bubble instead of paying diamonds?

That's exactly as intended. The idea is that you must pay to unlock the item from the bubble, else you lose it. When the bubble pops, it leaves behind energy: the item is then gone. In my experience it's usually never worth the expense to try and unlock bubble items, plus the energy adds up fast if you save and merge them to level 5 (100 energy) - that's free energy!