Theoretically possible, but I think that it's also possible to find a solution. Beyond that it's at the moment and in the nearly future very hard and expensive to get a quantum computer and besides that a quantum computer must be developed for the purpose of destroying bitcoin (to solve SHA256), with a normal quantum computer, you don't have the "instruments" to solve SHA256 you have to develop a special system for the quantum computer. In my opinion the effort is too big for most of the people. Furthermore, quantum computer's can also be used to attack other things like bitcoin nearly everything could be hacked, and there are some big banks for instance, which would be more profitable and easier to attack.