puzzleSolution

Sequence of states from the current puzzle state to the goal state.

Since the 8-puzzle grids generated in this app are guaranteed to be solvable (via the getValidShuffledState() method in ShuffleUtil), the only time when the value of this stack is null is prior to the first time the Show Solution button is clicked.

Its type is set to nullable only to conform to the data type of the return value of the solve() method in the SolveUtil class.